The ability cap refers to the maximum bonus applied from all magical and bonus effects. In NWN2, only the highest strength effect applies, wearing both a +4 belt of strength and a +2 ring of strength would give +4 strength, not +6. (The same is true for other abilities as well, such as INT, DEX etc.). The exception to this is Barbarian Rage, (not spell Rage), which stacks with other ability bonus. Bonuses from the Great Attribute feats (e.g. Great Strength) and the Red Dragon Disciples attribute increases are considered natural attributes (they can be used to meet pre-requisites for feats, they stack with bonuses from items and they don't count towards the +12 cap).
- There is a literal ability cap for all effects that modders should be aware of. The ability score cap is +12.
- Another non-ability cap to be aware of is the dodge AC cap, which is +20. Dodge bonuses stack, and there are many in the game.
- As for the cap on Ability Scores themselves, NWN2 has a hardcoded ability score cap of 50 (+20 modifier). Any score above 50 will not work properly and will, in most cases, be treated as a score of 50.
Changes from NWN1
Ability cap has changed from NWN1. Instead only the highest ability modifier for an ability is used. So if you have a belt of frost giant strength (strength +4) and an weapon which gives you +2 you would gain a +4 increase to strength. Under the old rules you would have gained +6 to strength (the magic ability bonus was capped at +12 hence the name).
In 3.0/3.5e enhancement bonuses do not stack. Items and spells which grant enhancement bonuses to a particular stat used to stack in NWN because they did not choose to implement this rule. NWN2 correctly implements the rule. Making a +6 stat enhancement item the pinnacle of a stat increase. However there are other types of temporary bonuses (e.g. Morale bonuses, like Rage and Frenzy, Sacred Bonuses, Profane Bonuses, Insight Bonuses) which do stack with enhancement bonuses. In general you get the the combined effect of the highest bonus of each type to an attribute. Different types of ability bonuses were not implemented in NWN2, so there is no stacking except for Rage.