In Overlord the Cargo Cruisers are the craft used to transport resources between your worlds. Early on in a new system you will rely on these ships to maintain your budding empire until you can stabilize. Cargo Cruisers come in two sizes; Class A holds twice as much as Class B, but it also uses twice the fuel, costs more, and deteriorates faster which allows fewer uses. You gain access to the Class A type after the Hitotsu System. Apparently any resources can be stored within Cargo Cruisers indefinitely; people won't die there nor will food spoil.