Tweendeckers are general cargo ships with two or sometimes three decks

Deck (ship)

A deck is a permanent covering over a compartment or a hull of a ship. On a boat or ship, the primary deck is the horizontal structure which forms the 'roof' for the hull, which both strengthens the hull and serves as the primary working surface...

. The upper deck is called the main deck or weather deck, and the lower deck is the tweendeck. Cargo such as bales, bags, or drums can stacked in the tweendeck space, atop the tweendeck. Beneath the tweendeck is the hold space, used for general cargo.
Cargo ships that have fittings to carry standard shipping containers

Intermodal container

An intermodal container is a standardized reusable steel box used for the safe, efficient and secure storage and movement of materials and products within a global containerized intermodal freight transport system...

and retractable tweendecks (that can be moved out of the way) so that the ship can carry bulk cargo are known as multipurpose vessels (M/V).