Currently, armour stacks multiplicatively. A person wearing a SCAF suit with the helmet up, as well as a watchdog respirator, for instance, would calculate their effective piercing resistance as such:
0.55x0.95x0.95=0.4966
45% piercing resistance from the SCAF suit, 5% from the SCAF suit helmet, and 5% from the mask.
AP applies as an exponent to effective resistance, and is calculated by subtracting the AP value of the projectile from 1. Something with 0.7 AP, for instance, would result in an exponent to the resistance of 0.3. For the SCAF suit example earlier, this would be 0.4966^0.3, or around 0.8106 (around 19% resistance). This resistance applies to the damage of the projectile itself.
These systems are completely unintuitive, hard to caluclate on the fly, and fall apart completely when armour plates are added to the calculation. A plasteel plate has 75HP before it breaks. It doesn't have any resistances, do AP does not apply. A 9x19mm AP round does 7 piercing damage. An FMJ round does 24. It takes four FMJ rounds or eleven AP rounds to break a plasteel plate. That's nearly three times as long.
And even without taking plating into account, the current damage of AP rounds is so low that it doesn't even matter anyways. As seen in the photo attached, which is my manual TTK testing, of which the 9x19mm AP round has a whopping 0.9 AP, it falls behind FMJ in every single metric, taking four seconds longer to kill a person without a plate than a person WITH one, if shot by FMJs. The only situation in which I have found that AP increases TTK is with the juggernaut suit, which has 80% pierce resistance. It took 18 shots AP 9x19mm versus 22 shots FMJ 9x19mm to crit, and even then, adding a single plasteel plate brings that to 29 shots AP and 26 FMJ.
There is no reason to use AP ammo under any circumstance.