A Mobile Resupply ability is a Specialization in many installments of the Battlefield series. The ability allows players to resupply nearby teammates with ammunition, similar to the Ammunition Box, when the player is occupying a vehicle.
It increases the rate at which ammunition and supplies are resupplied from the Ammo Hub, and also allows a Support soldier inside a vehicle to resupply infantry outside the vehicle, as well as other vehicles.
SUPPLY UNIT is a specialization featured in Battlefield 4 and is the fourth specialization apart of the Indirect Fire Upgrade Path. It causes occupied vehicles to issue additional ammunition to nearby soldiers.
— In-game description