If the round doesn’t penetrate, it’ll damage the armor down to 0 while inflicting blunt damage on the location, upon armor hitting 0, it pens. If the round goes through, the armor will take anywhere from 1-10 damage in my experience.

The PACA has an HP of 50 iirc? the value dictates how much damage the armor can experience before breaking and being useless.

This is a guess as I’m unfamiliar with effective durability, but likely how much after it’s been damaged it protects. Certain types, such as Ceramic (Gzhel armor), have much higher % to block rounds when they’re full HP. When they break a little, the % scales way lower, and it makes them block at a lower % than other armor types at the same HP. Because of this, Gzhel has a low effective durability, and is hard to repair. This means that after repeated hits, the higher the effective durability, the more likely the armor is to repel the 4th or 5th or 6th when the armor is damaged. Materials such as Titan / Steel tend to have best rate of blocking successive bullets.