Armor pieces are clothing accessories that provide protection from attacks. Armor for specific areas of the body occupies the Accessory slot(s) for that area.
Each point of armor a creature has prevents 1 point of damage from all attacks. Natural Armor can't reduce the total damage from an attack to less than 1. Equipped armor is always calculated before natural armor. Some damage types ignore natural, equipped, or all armor.
Natural Armor is the innate protection a creature's body has from its various structures protecting vital organs. A character's natural armor rating is determined by their race. Humans have natural armor equal to their size if it's positive.
Equipped Armor is the Armor Rating of a worn item covering a specific part of the body. In most cases, only one piece of worn armor will apply to a given attack depending on the part of the creature's body that gets hit. Some creatures are smart enough to attack unarmored parts of their target. See called shots.
Whenever a piece of armor absorbs damage from a critical hit, or the damage from an attack is greater than twice its armor rating, the GM makes a durability check for that piece of armor. If the check fails, the condition of the armor deteriorates by one level. The GM may call for durability checks against armor at any time they feel is appropriate, whenever an event might cause a character's armor to become damaged.