Tankers primarily transport various liquids or gases, oxygen, nitrogen, water, fuel, chemicals, etc.
In addition to transporting and distributing these substances, many of the larger tankers can refine the chemicals and elements directly from the atmosphere of a planet. This can be a slow, tedious process if their are many impurities, but it is a capacity that has kept the Fugitive Fleet supplied with fuel, air, water, fertilizer, hydro carbons and many essential materials.
Most tankers are able to section off, process and store multiple substances.

Transport Tanker

Refinery Tanker

Refinery Tanker