Defence bonuses are added to toughness, along with endurance and scale bonuses. The most common form of defence boost comes from worn armour. There are several other types of Defence, however:
Natural Armour: This is an amour bonus that comes from having thick skin, scales, bony plates, etc.. Creatures have a +0 natural armour bonus unless otherwise specified, which is why armour bonuses lower than +0 can‚t decrease your toughness. If you are wearing a paper shirt (defence …bonus‚ -3), you don‚t decrease your toughness. Your natural armour takes over, with its +0.
Durability: This bonus comes from the material an object is made out of. Living creatures usually do not have a Durability Defence bonus.
Unarmoured: This bonus is granted by the Unarmoured skill.
Stacking Bonuses: The bonuses from different Defence sources do not stack.