I actually think it should be such that any mod can go on any item of it’s type, but certain mods have elemental affinity, which reduces the energy needed to attach the mod to the armor. So say, shotgun scavenger takes 4 energy on grips normally, but if you match it to an arc grip it only costs 2 or 3. This way players can choose whether or not they want to use the extra energy on an unmatching element, instead of being barred entirely from the perk due to bad rng. It also lets you have and use your favorite perks while still looking for that perfect match.