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Penetration
Penetration is the ability for hits that do elemental damage to ignore a proportion of the target's resistance to a specific element. It comes in fire, cold, or lightning. A source that offers elemental penetration gives all three.
Mechanics
Penetration causes the specified amount to be subtracted from the target's corresponding resistance when determining the damage they take from a hit, to a minimum of 0% resistance. Since penetration only affects hits and applies to the target's defensive stats rather than your own offensive stats, it does not affect damage with ailments.