Similar to a soldier's Inventory, trunks are the compartments in vehicles for storing ammunition and supplies. Most vehicle weapons are fed directly from the trunk, with the exceptions being the Flail and Switchblade, which use on-board capacitors for their ammunition.
Trunks are of varying size in relation to the vehicle's mass and function.
Vehicles with weapon systems have a default loadout of ammuntion for their guns loaded into the trunk on creation, but these contents may be changed quickly at a Repair/Rearm Silo, Landing Pad, or Lodestar. These terminals also allow depleted ammunition to be instantly restocked in the trunk, via the vehicle Favorites menu.
In emergencies, i.e only Towers and Advanced Mobile Stations available, the trunk may be manually restocked by the driver using the 'Vehicles' tab at any standard Equipment terminal, and carrying the appropriate boxes back to the vehicle's trunk.
The trunk may be used to carry any items at all, not just vehicle ammunition, and as most ammo boxes are 4x4, there is usually room for extra items down the sides: extra Armor Canisters, Adaptive Construction Engines, etc. These items will save into any vehicle Favorite preset along with the ammo, provided you have the appropriate certification.
(Note: The trunk may be locked like the Driver, Gunner and Passenger seats; set to Driver only, Squad Only and Empire - remember to unlock it if you want your gunner to do a manual reload.)