Multi-layer armor applies its armor value for each layer it occupies.
How effective this is can be configured in the settings. Recommend ~50% and reduce armor penetration
In vanilla, Armor is applied to an attack for each layer of apparel separately. Marine Armor takes up two layers, but its armor is only applied once. A flak vest and jacket could equal marine armor (for sharp attacks on torso with low armor penetration).
This mod simply treats multi-layer armor (like Marine Armor, which occupies middle + outer layer) as two separate layers of armor, so its armor applies twice to every attack. A 50%(configurable) rating multiplier is applied to the second layer, and that seems about right for a ~40% buff.