Layer /should/ hit top layers first. If it is working.
Lets take you as an example, wearing your single trench coat on top, with a tank top below that, and finally an under armor.
Lets say something hits you (not too big of an assumption). They hit the torso, and so the game uses the coverage amount of each individual piece of clothing to find out what it hits.
IIRC a trench coat has 95% coverage (I could be wrong), so the game rolls the dice and sees that the shot hits you in the 5% that the trench coat is not covering. Then comes the tank top, no clue about the coverage value, but the game rolls the dice again and sees that the punch does hit you where your tank top is covering your torso, therefore some of the damage is transferred to your tank top.
Back on topic …