The truck with a covered bed, which is located among the game files, was not used anywhere.
It has room for two people. The bed is covered by a high-anchored white tarpaulin with the inscription "Atlantic Import", the rear part of the bed is uncovered.
The truck was created based on a real Ford Model AA truck. It can be obtained through a trainer, modification, using scripts in the command console or by swapping models in the game files. The model with the truck is called truckbxx00.4ds. Considering the aforementioned inscription "Atlantic Import" on the tarpaulin of this truck, I would say
that it could have been part of the chapter Just for Relaxation, in which
we use a slightly different truck to rob heavily guarded crates from the port. Paulie even says in the cutscene that the truck has the words "Atlantic Import" on it, the one we actually use in this mission, but it has the text "Atlantic Export". There is another model of this truck in the game. It is exactly the same, but it has an unfinished ramp in the back.