Can be both a flat number or a percentage. Ignores x or x% of target's armor. If the item has both flat and % scaling armor penetration, then the % is applied to the full value and then the flat penetration is subtracted from the result. Example: Sword has 10 armor penetration and 25% armor penetration. Target has 200 armor. 25% of 200 is 50. 200-50=150. 150-10=140. The sword ignores 60 armor points of the target. Dealing damage to the target as if they had 140 armor instead of 200.