Stacking is the concept where more than one instance of the same type of effect is cumulative, possibly up to a cap. The rules that determine when things stack are somewhat haphazard, with many exceptions. Some of these take into consideration the source of an effect.
Examples of things which do stack:
- Ability score bonuses stack up to +12.
- Armor class bonuses of different types stack.
- Dodge bonuses to armor class stack up to +20.
- Saving throw bonuses stack up to +20.
- Skill bonuses stack up to +50.
- Damage immunity stacks.
Examples of things which do not stack:
- Armor class bonuses of the same type, other than dodge, do not stack.
- Multiple uses of the same spell, such as bull's strength, do not stack (but the spell does stack with the bull's strength feat as those are technically different spells).
- Concealment does not stack.
- Damage reduction does not stack.
- Damage resistance against the same type of damage does not stack (except for resistance from feats).
- Spell resistance does not stack.