This FAQ is to address somewhat more detailed gameplay questions and topics.
All units will receive resupply of fuel and ammo when in 'Supplied Territory'. The Supply Level is shown as a strength bar in some of the Land Department panels, and a Supply Level Overlay can be toggled on the map using the hotkey 'S'. Units receive supply when in their own territory, and when in the territory of a region that has provided a 'Transit and Supply' treaty appropriate for that type of unit (Land/Sea/Air).
Units outside of Supplied Territory can also be resupplied by a transfer from a 'Supply Unit' (Supply Truck, Transport Plane or Helicopter). Naval Units (surface and sub) can be resupplied by a Transport boat or by a Transport Helicopter, but not by fixed-wing Transport Planes.
Some air units are also 'Air Refueling capable', which means that they can receive more fuel (but not ammo) from Air units that have an 'Air Tanker' specialty.