The Armed Truck accumulates idle rewards. It requires Base level 5 for construction and has a single building level. Once available, it is a source of rewards you return to collect between activities.
How idle rewards work
Rewards build up over time until the accumulation limit is reached. Leaving the truck full prevents further accumulation, so the amount collected after a long absence is limited by capacity as well as time.
The game can notify you when the Armed Truck is fully loaded. Collecting takes the waiting rewards; it is a different action from upgrading a building or dispatching a squad.
Output and accumulation time
Armed Truck Resource Output and Reward Accumulation Time are separate bonuses. One concerns the supply of rewards, while the other concerns how long they can build up. Neither is an additional march slot.
The Trade Truck handles transport with an assigned escort. That is a different activity from Armed Truck idle rewards. The Alliance Trade Train has its own shared journey and passenger system.