CAP493 (MATS part 1) section 1 chapter subsection 10,
Category Type of Flight
A Aircraft in emergency (e.g. engine fault, fuel shortage, seriously ill
passenger). Aircraft which have declared a 'Police Emergency'.
Ambulance/Medical aircraft when the safety of life is involved.
B Flights operating for search and rescue or other humanitarian
reasons. Post accident flight checks. Other flights, including Open
Skies Flights, authorised by the CAA. Police flights under normal
C Royal Flights Flights carrying visiting Heads of State which have been notified by NOTAM/Temporary Supplement.
D Flights notified by the CAA carrying Heads of Government or very
senior government ministers.
E Flight check aircraft engaged on, or in transit to, time or weather
critical calibration flights.
Other flights authorised by the CAA.
i) Flights which have filed a flight plan in the normal way and conforming with
normal routing procedures.
ii) Initial instrument flight tests conducted by the CAA Flight Examining Unit.
(RTF callsign “EXAM”)
Z Training, non-standard and other flights.
And so priority is granted by status above, so catA versus catB, A gets to do what they want first, then B, and both before normal flights.