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.
When the above bonuses come from spells, the bonuses stack if they come from different spells. (So the wisdom bonuses from owl's wisdom and owl's insight stack.)
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.