I consider criticals and specials to be exceptionally lucky and/or powerful blows.
With random armour you get a dice code to represent your whole coverage, but often you will get the following two things coming into it.
1. Wearing a helm often gives you a +1 or so to overall armour. EG: Chainmail d6+1, Chainmail with helm d6+2 (not correct figures but you get the idea)
2. Sometimes the severe wounds chart says things like. "If you are wearing a helm X happens, if you are not severe version of X happens"
Luck,