Category "D" is the flight priority of that aircraft. A category D flight has priority over a "normal" flight.
The list of categories used are:
A
B
C
D
E
Normal
Z
Taken from the book:
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.
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.
NORMAL FLIGHTS
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.
Hope that helps
