Aluminum Plate is a gear perk that greatly improves your defense against soft-target rounds, at the cost of making you slightly noisier when you move. It is a solid pick against FND troops using that ammo.
Enemies firing soft-target ammo do much less damage to you. In exchange, your footsteps are a little easier for guards to hear.
Wear one of the gear pieces that carry it: Soft-Target Boots MK II, Space Marine Fatigues. Each piece only takes one body slot, so plan around the other perks you want.
Worth it for open combat against soft-target users. Skip it on a stealth run where the extra noise can blow your cover.