Trade is a unit command that will allow two adjacent units to trade items between each other. This is particularly useful if ever one unit needs an emergency supply of recovery items or intact weapons that they can use.
The Convoy command in the other hand can be used by any allied unit as long as they’re adjacent to Byleth. Byleth can access this command anywhere. This allows users to access the army’s stockpile of weapons and items to trade or store items there. Moving items back and forth from your inventory to the convoy will not consume a turn. You can still use the Trade command to transfer items to adjacent allies.