You can distinguish between actively and passively cooled units.
Conair containers, which are now almost no longer in use, are brought to low temperatures via external cooling rods. The external cooling is achieved by the introduction of cold air from the outside.
Today’s integral containers have an active cooling system based on a built-in cooling unit. This unit can be operated with electricity or via a diesel generator.
In addition, it is possible to operate a reefer container without a a cooling unit as an insulating container. In particular, former Conair containers are used so that the porthole openings are sealed with insulating panels. Integral containers with defective cooling units can also be used as ISO containers until they are repaired.
These insulating units are able to keep a constant temperature and so that delicate perishables or machines are protected from outside influences.