Do damage reduction types stack, or do you merely get the best one? For example, a paladin wearing adamantine full plate armour with the armour specialisation - would he get DR 3/adamantine, or DR 5 with 3 points being negated by adamantine?
If a creature has damage reduction from more than one source, the two forms of damage reduction do not stack. Instead, the creature gets the benefit of the best damage reduction in a given situation.