So; with with changes from states to Governing Capacity in 1.30; I thought it a good idea to talk about subjects and write a little guide about them. Since a lot of people IMO heavily underutilise subject nations, particularly in Single-Player. [A subject-focused build is generally worse in MP because you generally want to avoid Diplomatic-focused ideas in MP; or relying on the AI; although you can certainly make use of it].
For the purposes of screenshots; I'm just using console commands and tagging between nations in 1444.
Types of Subjects - At a glance
There are 4 main types of subjects in EU4. Vassals; Marches; Personal Unions and Colonial Nations. All types of subjects use Liberty Desire; which you will generally endevour to keep below 50%; as past 50% the AI will stop paying any taxes; generally not help in wars, and becomes able to ally with other powers [And other disloyal subjects] to try and declare independance.
Generally; the order of most rebellious to least rebellious is: Vassal >> March > Colonial Nation > Personal Union; although the latter three are relatively close and can interchange in certain situations.
Vassals are the most simply type of subject; and easiest to form. They can be forced; diplomatically acquired; or released.
Marches are an alternate type of vassal, focused more on existing for a long-term and for military purposes.
Personal Unions are acquired by luck mostly; or missions. They are also exclusive to Christian nations.
Colonial Nations are formed when you own 5 or more provinces in a colonial region, and basically manage your provinces in the New World for you.
Liberty Desire - General Points
Liberty Desire is a measure of how much a vassal wishes for independence. 
Liberty Desire scales from many factors; such as vassal's relative power to you; opinion; trust; Diplomatic Reputation; and various effects and interactions. May subject types also start getting a scaleing Liberty Desire if they break 100 and 300 development. [Personal Unions being the major exception].
If a subject of any type breaks 50% liberty desire they will turn rebellious. This means that they will stop paying you any money; generally not support you in wartime [Although they will often defend their own territory] and most notably; unless they have a truce with you; they will be able to ally foreign powers or other rebellious subjects and try and declare independence. This last one in particular is painful as it can very quickly cause Liberty Desire to cascade to levels where a subject may never be loyal again.
General ways of reduceing Liberty Desire include keeping Opinion high; Trust [Builds over time; up to 20% LD reduction at 100 Trust]; Influence Ideas [-15% LD; plus 6% more from Diplomatic Reputation]; and Diplomatic Reputation [-3% per point]. There are also other modifiers; but several of these vary on types of vassal; or even game phases. [IE: There's ways to reduce liberty desire in the Age of Absolutism]
Also; notably; Iqta governments can choose Lenient Taxation to give all subjects a -15% LD and gain +1 Diplo Rep; effectively giving -18% LD. Throw in Iqta's +33% Tax from subjects and they are very well-suited to vassal play.
You can see most of the stats of your vassals in the interaction screen:
General Advantages and disadvantages of having subjects
+ You are not owning the land yourself; meaning you do not have to use your Governing Capacity for it, or spend ADM coreing it. Every individual nation has it's own Governing Capacity; and you'll generally get more out of vassals than Territories.
+ The vassal's own troops and manpower pool remain. For small areas; the base 10,000 manpower and 3 FL are likly more than you would have gotten from the land yourself earlygame
+ You do not have to deal with rebels, as the land has not been conqured. [Unless feeding a vassal] Likewise; autonomy will not be raised by making a subject
+ Every nation has it's own ruler; own 3/3/3 Monarch point generation and own event rolls. This leads to higher overall MP generation for your empire; and generally more development.
+ Vassal troops can be cannon fodder and save your own finances and manpower pools
+ Subjects increase your own Force Limits
+ Vassals can hold wrong culture/wrong religion land for you. You can even use your missionaries; with their modifiers; to convert your vassals land to their religion
+ Costs DIP instead of ADM to make into your cores
+ More reliable than allies; they will always join your wars [barring Scutage] and they will never ally your enemies despite being allied to you; or suddenly get -200 wants your provinces modifiers.
+ Force-Vassalisation generally produces less Aggressive Expansion than Annexation.
- AI is AI. Their development, troop movements; buildings and idea selection will not be optimal.
- Chance of rebellion if poorly managed
- If you want to commit to vassals, you need to build for it. Tiny vassals simply won't be relevant lategame.
- Without Espionage; they can get in the way of your claims; and who wants Espionage?
- Subjects will eat trade income
- Subjects use a Diplomaic Relation slot
- Subject countries will NEVER choose Exploration Ideas [Although they will fill and use them if already selected]. Even a subject Portual/Castile/Spain will never choose them despite having a x1000 modifier to take them. Only Colonial Nations will choose Expansion Ideas.
The baseline mechanics for both of these subject types are rather similar, but they do differ. Vassals and Marches are both primarily acquired by warfare. Either by force-vassalising a country outright; or releaseing cores of a conqured country. You can also Diplomatically-Vassalise small countries; but this is generally limited to 1~3 Province minors; who are outside the HRE, and is generally only viable for major nations. I would advise against a force-vassalisation in most cases rather than an annex-release. Annex-Release removes negative Trust and Opinion modifiers from a Force-Vassalisation, although it does produce significantly higher AE in the HRE. Vassals can be annexed after 10 years of being created; with opinon at 190+; and the process being started while at peace. The process will continue while at war however. Larger vassals take longer and cost more to annex. A vassal will add 1+10% of it's Force Limit to it's overlord by default. Vassals by default have a base Force Limit of 3 [Free nations have 6]. This is why different Vassals can give different amounts of FL. For example; in 1444 Khorasan has a FL of 4; so gives Timurids a 1.4 FL boost. Trasoxiania however has 15 FL; so gives 2.5 [1+1.5]. Vassals are usually the most rebellious of subjects, as they have the 'Strength of all subjects compared to Overlord' modifier; and no special reductions. They pay a percentage of their tax income to the Overlord as well. Vassals should usually be used to avoid rebellions, reduce AE, and hold land. If you intend to keep them long-term; generally one should turn them into Marches. Note that while flipping from Vassal to March has no ill effects [except a loss in tax income]; going from March to Vassal gives you a -1 Stabhit and a relations drop. Vassals also have the unique option of Scutage. This stops them being called to wars but makes them pay an extra 50% tax to you. This can notably be used [together with vassal feeding and releaseing Normandy] to cheese the 100 Years War; createing a neutral buffer in Normandy that France cannot invade but you can land troops in.
Marches If there's a bunch of land you do not want to personally hold; you can absolutely give it to a subject nation and make it a March. Often, this can end up being more efficient. Especially when you consider that as long as a March is 25% or less of the Overlord's development; they gain significant military bonuses: +25% Manpower
Marches are what I regard as long-term vassals. They gain an increased military ability; but stop paying you taxes. This small loss in income is generally more than worth the benefits that a March gives you, although Marches cannot be annexed. Marches also inherently gain a -15% Liberty Desire bonus.
+30% Naval and Land FL [This feeds into your own FL]
+20% Fort Defense
-20% Fort Maintenance
+20% Manpower Recovery
-20% Land Maintenance
-20% Naval Maintenance
Furthermore; despite the size of the March; March Force Limit contribution rises to 1+20%; almost double that of a Vassal. They also gain some more; very powerful; unique interactions.
As shown before; Khorastan had a Force Limit of 4; and was giving 1.4 FL to Timurids. Changing Khorastan to a March however:
Increases their own FL to 6; and their contribution to 2.2FL; almost as much as Trasoxiania with 15FL.
In addittion; there is the unique ability 'Send Army Officers' which will cost you 10% of their army maintenance to grant them a massive +20% Morale and 5% Discipline; making virtually any March a decent army in quality. To boot; this gives -10% LD [Effectively giving Marches a -25% LD over vassals]. Should you have a surplus in manpower; you can sand 10% of your monthly manpower to your Marches to refill their own.
Obviously; to make the most of this; you will want the money to pay for Send Army Officers; and the money to pay for your own increased FL. This makes Marches a powerful option for richer nations. Merchant Republics in particular like using Marches; being focused on income to begin with and having increased Governing Cap costs for holding land themselves. A Merchant Republic can focus on making money, make a swarm of Marches and fund them to roll over it's enemies for them.
Vassal FL Contribution; and scaling ofWhile it is also relevant for the remaining subject types; I feel this is the best time to talk about this modifier. While it was bugged for a very long time so that it only worked with the base +1 [And previously +2 for Marches] FL contribution subjects gave, making the modifier borderline worthless, Vassal FL contribution is an exceptionally powerful stat in a vassal build. Using the console to bump up Timurids tech level to 5, and taking Innovative Ideas [Which you should always take if you are playing with Vassals] does this: Now; jumping into Transoxiana for a moment, I will bump them up to Tech 5; and give them Quantity Ideas; raising their FL to 23.
That's a gain of +9 FL for Timurids; which is about a 33% increase. Note this is with only one subject as a March, and in 1444. With this Influence Idea, the Vassal FL Contributions are now 2+20% for Vassals and 2+40% for Marches. Notably; this is on top of the other benefits of Influence such as -15% LD; and +2 Diplo Rep.
This not only raised Transoxiana's FL from 15 to 23 [8 more units] but gave Timurids +1.6 FL [20% of 8]. Note this means any FL modifier in a subject is effectively passed onto you. For having subjects with +FL modifiers can be particularly powerful; especially Marches.
Every country can also take 2 more policies which give another 100% Vassal FL contribution each. Economic+Influence; and Economic+Plutocratic/Influence. Plutocratic is especially notable that it gives you another -10% Liberty Desire as well; meaning Pluto and Influence combined are -25% Liberty Desire and +200% Vassal FL Contribution.
Throwing these onto Timurids [Which you can take as your first 3 idea groups btw] turns their FL into this; which is 4+40% and 4+80%:
That is 4+40% for Vassals; and 4+80% for Marches. At this point; it is probably more effective to build Land FL buildings in Marches than yourself; as you still gain 80% of the increase, while the March also gets 100% effectiveness. In essence, this makes a +1 FL building +1.8 FL.
Oh; and these bonuses also scale with your own FL modifiers. Giving Timurids Quantity Ideas after this:
Increased FL from 61 to 92. So with a March with Quantity; and you having Quanity; you can get an absurd stacking multiplier to any sort of Force Limit increase to your March; as their final; post modifier FL is what is added to you as a flat amount, which is then increased by your own modifiers. It double-dips. At this point Khorastan [March] is also giving more FL than they actually have.
You can actually get to +400% Vassal FL contribution modifier if you take Aristocratic or Plutocratic; and then reform and take the other one. This will give you 5+50% and 5+100%. The best way to accomplish this is as a Tribe, as they can take Aristocratic and then reform into a Republic earliest. A few nations [Bosnia; Ashikaga; Andalusia and Herzegovina] also have +100% Vassal FL contribution in their national ideas; allowing them to get to 6+60% and 6+120% [Meaning every 1 FL in a March gives you 1.2].
For absurdity's sake; here's the Timurids turned into a Republic; with Plutocratic Ideas as well; although at this point you're probably more effective going for quality yourself; it still added another 16 FL. Remember, this is 1444. This can easily be scaled further by building FL buildings in subjects, increaseing subject development, subjects taking Offensive/Quantity Ideas, and making the other subjects Marches as well.
I can assure you; if you were to annex all the land directly; Timurids would have nowhere near 108 FL.
Personal Unions
Personal Unions are a type of subject only Christian countries can have. The main ways of obtaining them are: 1: Events/Missions; usually via a 'Restoration of Union' CB; such as the 100 Years war with England and France; the Iberian Wedding with Castile and Aragon; and Poland over Lirhuania
2: Starting with them, IE: Denmark and Sweden.
3: Luck; by having a Royal Marriage [or a shared dynasty in some cases] with a nation with no heir, in certain conditions. I am no expert on PU mechanics however.
4: Challenging a Succession War and winning
Personal Unions take 50 years to be able to be integrated. After 50 years, a Personal Union can be inherited outright; immediately and for free; on ruler death with an RNG roll; which scales down with province count and up with Diplomatic Reputation.
Personal Unions will break on ruler death if the subject has a negative opinion. In addittion, while they do suffer from a 'strength compared to overlord' Liberty Desire malus; they do *not* scale with a 'strength of ALL VASSALS modifier; only their own:
However; a Personal Union will gain Liberty Desire if their overlord has negative Prestige. They will also become independant if Pretenders take over their country, so make sure you swat those. However, if you can manage your Prestige, Personal Unions are usually less rebellious than Vassals.
Notably; Personal Unions do NOT gain Liberty Desire from their development; like any other subject. This means you can feed them to your leisure; and you can have massive Junior Partners. In fact, if you get a PU over a large nation that you can hold onto; it's probobly best to not intergrate them due to the massive DIP and time cost; and just use them as a battering ram and to save yourself ADM points. This is especially notable for countries which can get multiple; large PU's. Looking at you; Habsurgs.
A lategame Hungarian PU with over 500 development, with no added Liberty Desire. Note I do not even have Influence to keep them in check and they still have 0% LD.
However; Personal Unions do NOT increase your own Force Limits:
Note that Brandenburg and Riga are on my Bohemia's list; but not Hungary. However; Hungary has 172 FL in it's own right, and is saving me over 500 governing cap of nations in the wrong culture group... so it's not exactly bad to have around.
This also means if going for a Game of Thrones, you shouldn't bother with stacking Vassal FL Contribution. You should instead go for Diplomatic Ideas, which allow you to break off RM's without a stabhit.
Also; Aragon and Lithuania are PU's that come with a 'Historical Friends' modifier by event; giving them a -50% LD. This makes them almost impossible to lose to rebellion unless you seriously mismanage them or your own country. Likewise; 'Historical Rivals' [Which Sweden can gain by event] gives a +50% LD modifier; making them very difficult to hang onto.
Due to the inheritance mechanic; it's generally not worth integrating Personal Unions. Small province-count PU's can just be inherited; while larger ones are generally strong enough to want as a permanent ally in the first place. The exception may be low-development but high-province-count subjects like Norway. Also; Lithuania and Aragon can be integrated via Decision for free as you form PLC and Spain.
Colonial Nations
Colonial Nations are another form of subject; which form when you own 5 for more provinces in a colonial region. These nations will pay Tariffs to you [a separate modifier to vassal tax; and can be raised or lowered but will impact Liberty Desire] and transfer 50% of their trade power to you. They; like Personal Unions, will only scale from their own relative power to your nation for Liberty Desire; although they will scale with development. They will also gain Liberty Desire from Mercantilism and Administrative Efficiency [Representing these nations gaining a drive for independence as time progressed]. This generally makes them less rebellious than Vassals [Unless you jack up Mercantilism and Tarriffs] but more rebellious than PU's. There are also events which can temporarily raise their Liberty Desire. Librerty Desire from development usually dosen't become a large issue for Colonial Nations due to low base development, although it can become high if the CN develops a lot [If they become rich this can easily happen by them stacking modifiers and hireing advisors] or if you feed them half of North America.
An example of a CN with over 300 development; which gives a hefty LD penalty. You can also see Trust; Mercanalism and Tarriff modifiers. Don't ask why I'm a Malaya who owns the 13 colonies. I totally didn't convert England to Javan and form Malaya to get the Spice Must Flow in the stupidest way possible.
Colonial Nations will; after hitting 10 Provinces; give you +1 Merchant, +10 Naval FL; +5 Land FL; and +5% Global Trade Power. Colonial Nations will also always take Expansion Ideas. This grants them 2 colonists. The recent buff to Expansion to give 2 colonists is a big buff to colonizers, as they simply need spawn a CN; feed it enough subsidies to afford it's own colonists; and they will start building their own colonies [or developing existing ones]. In tandem with Exploration only giving 1 colonist now; it essentially makes colonizing the New World slower to start but faster later. Colonial Nations will join you in your own wars. In addition, nations in the New World can declare war on other New World nations without calling in Overlords. This usually won't naturally occur too often; as Colonial Governors rarely roll Militarist, and even then the AI tends to be reluctant to extend outside it's own colonial region. There is actually a subject interaction called 'Start war in colony' which allows you to force them to start a war they have a CB for, with them as Warleader. Note however, it is quite likely that after a period of time the attacked CN will call in it's overlord; or have it's overlord intervene. If this occurs however, you get a CTA as well. This can actually be a good way to get a war on a nation without fighting it's allies; as the Overlord will enter as a
This quirk can also be abused as a nation with it's capital in the New World [ie: Aztec]. Especially if you can blitz the target to -50% Warscore. [Nations cannot join a war with warscore below 50%]
Trade Companies
These appear on the subjects screen. They are not subjects; although there is an interaction which can be done on this screen; giving you an increase in annual inflation but a boost in trade power from TC provinces in that region.
tl;dr
If you intend to keep a vassal long-term; make it a March March builds can be very powerful if you invest into them; especially as a naturally rich nation Personal Unions are usually worth keeping around unless you need the Diplo slot; or they are wide and poor; as they'll either auto-intergrate or are just generally good low-LD; hyper-loyal beatsticks. Use Vassals/Marches/PU's to hold land less efficient for you to hold directly. So; next time you want to blob out and paint the map; perhaps consider painting it a technicolor instead.
