Pen

There are certain weapons and damaging effects that are not just damaging in a particular way, but are also more penetrative in that damage type.

The most common damage type that can have this penetrative effect is pierce damage. Armour piercing rounds of some firearms are a classic example of this. The listings for these carry a penetration (pen) property.

The number after the pen property reduces the effective armour resistance to the weapon’s damage type by that number. Taking the example of a discarding sabot assault rifle, we can see that its listing has the properties and dmg(hp,prc)=3d6 : pen=5.

If you have armour with the property rst(prc)=10, then, against this weapon, the armour is effectively rst(prc)=5. It is reduced by 5 because of the pen=5 property.

The result is that this weapon’s attack’s damage would be reduced by 5, not 10, as if the armour’s property were rst(prc)=5.

If you have armour with anything up to rst(prc)=5, then this attack’s damage would be fully applied. No reduction at all. This is because the penetration of the weapon overcomes the armour entirely, and the attack will go right through the armour as if it wasn’t there. Against this weapon, you would need rst(prc)=6 at a minimum to get any reduction at all in damage.