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I found this list of bonus types: "Ability", "Armor", "Circumstance", "Insight", "Luck", "Morale", "Natural Armor", "Profane", "Racial", "Resistance", "Sacred", "Shield", "Size"
Which of these types are applied to armor class from any SRD source?
I was able to find examples for: "Ability", "Armor", "Natural Armor", "Shield", "Size"
Are there any that I'm missing?
For “playable” races, common AC modifiers (bonus or penalty), from most common to least:
Every character adds their Dexterity modifier to their AC. This is not an “ability bonus” to AC—it is an untyped bonus equal to their Dexterity modifier. “Ability” is not a bonus type.
Almost every player character will have an armor bonus, because almost everyone will equip armor (or cast mage armor or equivalent). Only monks (and monk-like classes such as battledancer and ninja) are at all likely to lack an armor bonus, though even these often get one (e.g. bracers of armor).
Shield bonuses are also extremely common, though not nearly as common as armor bonuses. Many characters will not have one as using both hands for one’s weapon(s) is common, and usually recommended for those interested in attacking. At high levels, though, even dedicated attackers are fairly likely to get a shield bonus from an animated shield.
A +1 size bonus is fairly common (Small characters), but any other value is quite unusual, and the majority of player characters will have no size bonus (Medium characters).
Natural armor bonuses are somewhat unusual among playable races, but not rare as class features or magical effects (e.g. a transmutation effect toughening one’s skin). The necklace of natural armor is a common magic item that also enhances natural armor.
Deflection bonuses are gained from a lot of magical effects, as well as from common magic items such as a ring of protection.
Dodge bonuses are always to AC, as far as I know, and are found on a few classes and spells. The combat maneuvers “fighting defensively” and “total defense”—available to literally everyone and everything in the game—also grant a dodge bonus. And then there are the Combat Expertise and Dodge feats, which are core and commonly-required for various other things. So, in theory, dodge bonuses are very commonly available. In practice, you rarely see them because fighting defensively, total defense, and Combat Expertise are almost never good choices, and the Dodge feat is so awful that even if you’re forced to take it, and even though it costs you nothing to use it once you have taken it, you’re liable to forget it.
Also, notable: dodge bonuses stack with each other, unlike most other types of bonuses (circumstance and racial bonuses do too, but as covered below, I don’t believe either is found as a bonus to AC in the rules).
Cover is not a type of bonus, but it’s a common situational effect that grants a bonus (untyped) to AC.
For that matter, worth pointing out explicitly that some bonuses are untyped. The bonuses from Dexterity and cover mentioned above, for example. Untyped bonuses are notable for stacking with other untyped bonuses, which typed bonuses don’t do (generally; circumstance, dodge, and racial bonuses are exceptions here).
Beyond this, the various types of AC modifier get much, much rarer. It would be very difficult to rank the relative frequency of each, since it will depend a lot on the game world, and the particular players you have and the characters they’re playing. Thus, the remaining bonuses I’ll discuss are simply in alphabetical order:
“Ability” bonuses, again, are not actually a thing.
Circumstance bonuses are usually applied to skills—the only printed circumstance bonus to AC that I can find is from a single, obscure spell (holy star from Spell Compendium). However, the point of circumstance bonuses isn’t really for the printed options that produce them—though there are quite a few of those, again usually for skills. The point of circumstance bonuses is that these are the game’s recommended way for a DM to handle a situation where a character “should” have a bonus, but no rule covers it. The DM is instructed to hand that character a circumstance bonus for that—and if the circumstance says that AC should be boosted, then the DM should give a circumstance bonus to AC.
Also, if there are multiple circumstances affecting AC, it’s possible a DM could give a character multiple circumstance bonuses, and unlike most types of bonus, those stack with each other.
Enhancement bonuses directly to AC are extremely rare—I cannot actually find any, though I seem to recall there is at least one source of such a thing. However, enhancement bonuses to other AC bonuses are very common—all magical armors have an enhancement bonus to their armor bonus, all magical shields have an enhancement bonus to their shield bonus, the necklace of natural armor applies an enhancement bonus to the wearer’s natural armor bonus, and so on.
Insight bonuses to AC are unusual; a handful of obscure prestige classes and another handful of obscure feats offer them. The powerful and popular 9th-level spell foresight offers a +2 insight bonus to AC, though this is not why the spell is powerful or popular.
Luck bonuses to anything are somewhat unusual, and I cannot find any to AC in particular.
Morale bonuses are almost-always to active rolls—attacks, saves, etc. I can’t find any to AC.
Profane bonuses and sacred bonuses are basically the same thing, just evil and good, respectively. Many features offer a profane bonus if you’re evil and a sacred bonus if you’re good. There is no rule against having bonuses of both types, and there’s probably a way to do it, but that’s rare. Actually, they’re each pretty rare even separately; a few obscure prestige classes and spells offer them to AC, but not many.
Racial bonuses to AC probably don’t exist at all—a race with a bonus to AC will almost-always have a natural armor bonus to AC, and even in the rare case where that doesn’t apply, it probably becomes deflection or something else. I can’t find any evidence of an actual racial bonus to AC.
Resistance bonuses are always to saving throws. I don’t know of any case of any resistance bonus to anything other than a saving throw. And if I found one, I would say whoever wrote it made a mistake doing so.
This list is necessarily incomplete. The game includes thousands of spells and hundreds of classes, and sometimes they incorporate random, unique bonus types that aren’t used anywhere else in the game. Some of those unique bonuses may be to AC. The rules have no problem with this, because one bonus type is usually identical to another—off the top of my head, only armor, natural armor, shield, circumstance, dodge, and racial bonuses have special rules, the first 3 because they don’t apply against touch attacks, the last three because they always stack. (I suppose we can also mention untyped bonuses here again, which also stack—but “untyped” is not a type, it’s the lack of a type.) So you cannot expect to have an exhaustive list of bonus types. There can always be another bonus type you’ve never seen before.
Artificers may be able to make up their own new bonus types on the fly! The 4th-level infusion item alteration changes the bonus type that an item applies to a different bonus type, so “you could change a ring of protection +1 so that it provided a natural armor bonus to Armor Class instead of a deflection bonus,” and nothing in the infusion says that it has to be a bonus type found anywhere else in the game. So it may very well be able to change the bonus type to something the artificer made up right then and there. Even if that’s not allowed, per the previous point, there are a ton of unique bonus types out there that the artificer could use.
