An air support unit is a unit that supports air units by refueling and repairing them.
Air staging facilities, the Cybran cruiser, and the UEF Fatboy provide a platform where aircraft can land. Air units that patrol close enough to them will automatically land to refuel or repair battle damage. They can also be explicitly ordered to land on these units.
Aircraft carriers are also air support units. When aircraft are loaded into carriers, the carrier repairs and refuels them. The carrier protects the aircraft from attack, though if the carrier is destroyed, all units loaded inside it are also destroyed.
For a list of air support units, see: Category:Air support units. Note that air factories are not air support units. They can only construct aircraft, not repair or refuel them.