C = cutting, P = piercing, B = blunt. The most important one currently is blunt, since blunt damage knocks your enemy unconscious rather than killing them so that you can take them prisoner.
EDIT:
Also, so I've heard, heavier armors have better resistance against cutting damage than against other damage types. That's just what I've heard though, don't know if it's really coded into the game or if it's wishful thinking on somebody's part. <shrug>