Modifier Types and Stacking

A modifier provides a bonus (a positive modifier) or a penalty (a negative modifier) to a die roll.
Bonuses with specific descriptors, such as "equipment bonus," generally don't stack (combine for cumulative effect) with others of the same type. In those cases, only the best bonus of that type applies.
The only specific bonuses that stack are dodge bonuses, synergy bonuses, and sometimes circumstance bonuses. Circumstance bonuses stack only if they're provided by differing circumstances; if two circumstance bonuses caused by similar circumstances apply, they don't stack.
Specific bonuses that don't stack include competence, cover, equipment, morale, natural armor, and size.
If the game setting includes magic or other supernatural effects, there can also be deflection, enhancement, enlargement, haste, inherent, insight, luck, profane, resistance, and sacred bonus descriptors. None of these bonuses stack.
Any bonus without a descriptor (such as simply a "+1 bonus") stacks with other bonuses.
All penalties stack, regardless of their descriptors.