Forsaken Plate Armor flatly reduces damage from the corresponding damage type per projectile. This damage reduction bonus occurs before any %-based damage reduction bonuses are applied. Each projectile in a salvo will be reduced by that amount.
For example, if a weapon has a salvo of 3, the Forsaken Plate armor will flatly reduce damage per projective/salvo, so each of the 3 salvo’s will deal reduced damage separately.
To help put this into context, the Explosive Plate armor would be great against Cerberus Rockets because the Cerberus rocket has a high salvo and explosive damage. The Ballistic plate would be good against Reaver Chainguns for the same reason due to the Chaingun’s high salvo and ballistic damage.