From AirTanker website:
AirTanker will employ its own A330-200 type qualified pilots who will operate the aircraft when they are on the civil register. This group will include a number of instructors and examiners who will be responsible for the type and air transport line training of RAF pilots.
Civil pilots employed by AirTanker will also be Sponsored Reservists. They will undertake regular training with the RAF in both military and tanker operations. Sponsored Reservists can be “Called Out” by the Ministry of Defence for permanent service alongside regular RAF personnel to provide a surge capability.
What is a "sponsored reservist"?
Thanks