Cargo is any essential material or substance needed to sustain the crew and/or spacecraft during their stay in space.
In Gameplay, there are four types of Cargo: Oxygen Tanks, Carbon Dioxide Scrubbers, Water Tanks and Batteries. Each is depicted as a small, brightly-colored box, labeled as to its particular contents.
Once a module containing Cargo reaches space, the cargo can be swapped, moved or disposed of after selecting the Cargo/Consumables tab in the VIEW section.
Several spacecraft can be loaded with cargo during the rocket assembly stage, such as: Automated Transfer Vehicles, the Resupply Module, LOK Crew Capsules, LOK Docking Modules, Crew Capsules, the Crew Capsule Dockable, and several others. As a general rule of thumb, if the module can carry a crew, it will carry cargo as well. Stations will need to be supplied with new/fresh cargo periodically, as the previously supplied cargo is depleted/used.
Specifically, the four types of Cargo are :
Oxygen Tanks - Oxygen Tanks supply breathable air to the crew members aboard the space station/vessel. Without it, oxygen levels will drop (shown on the ENVIRONMENT tab as "PRESSURE") until the levels reach orange or red, and the crew dies, though this is not shown in the game and is simply presumed to be their fate. NOTE: Oxygen can also be supplied by an Oxygen Garden, which is another module that can be attached to the station.
Carbon Dioxide Scrubbers - Carbon Dioxide Scrubbers absorb carbon dioxide exhaled by the crew. Without these scrubbers, carbon dioxide levels increase aboard the station. If these levels rise too high, the atmosphere aboard the station becomes toxic and the crew members die. NOTE: Carbon Dioxide Scrubbing can also be conducted by an Oxygen Garden.
Water Tanks - Water Tanks are simply containers that hold water. In Gameplay, water is used by an Oxygen Garden to help plants grow, which in turn, the plants produce oxygen for the crew to breathe and removes carbon dioxide from the air.
Batteries - Batteries provide emergency power to a space station or space vessel, should it not be connected to an external power source, such as solar panels. Some vessels are more power efficient than others, so monitoring power levels is always important.
One important fact to remember about batteries in cargo slots is: If you remove all the batteries from almost any vessel's Cargo slots, the vessel no longer has power and is inoperable. Without electricity, it cannot operate its main engine or thrusters, use NaviComp, or perform activities such as undocking; making it effectively useless unless rescued by another vessel. This does not apply to spacecraft with built-in solar panels such as the Automated Transfer Vehicle and Orion Service Module.