Armor DMG Mitigation Penetration
When calculating the damage, the damage reduction provided by the defender's Armor is calculated at a lower value.
When calculating Hit Damage, if the attacker has Armor DMG Mitigation Penetration, the corresponding value will be deducted from the damage reduction provided by the defender's Armor.
Armor DMG Mitigation Penetration doesn't change the actual Armor.
Armor DMG Mitigation Penetration can reduce the damage reduction provided by Armor to a negative value and additionally increase the damage taken by the defender by the corresponding value.