There are some armor mods that give +x% damage bonus against enemies of high/medium threat level. However, when I equip them, the respective value in the character screen does NOT change, it stays at 0. I don't know if that is a UI bug and it actually gets applied, or if it just doesn't get applied, but it's a bug either way.

EDIT: The same applies to damage reduction against continuous damage / negative effects. That also stays at 0 (bottom value under armor in character values screen) when equipping a mod.