The damageReduction attribute defines the armorpoints for armor. The higher the number the more armor you get. A damageReduction of one will give you a half armorpoint. The maximum displayable number for armorpoints in Minecraft is 10 (20 half or damageReduction = 20; ). I think you can set it higher than 20, but it won't show up in-game at the armorpointbar.
Syntax: damageReduction = <integer>;