Special designations were by the wartime Royal Air Force  to indicate the role and armament of an aircraft. For example, in the Mosquito NF Mark VI, "NF" signified "night fighter." Similarly "F" was used to designate fighters, "B" was used for bombers, "TB" for torpedo-bombers, "GR" for General Reconnaissance, "PR" for photographic-reconnaissance and "FB" for fighter-bombers. The terms "LF" and "HF" indicated whether a particular fighter aircraft is optimized for Low-Flight (LF) or High-Flight (HF).

 

As for armament designation, in the example of a Spitfire Mark IIa, the "a" indicates that the aircraft is armed with eight 0.303-caliber machine guns. if the configuration code was "b," then the aircraft would be armed with two 20 mm Hispano cannons and four 0.303-caliber machine-guns. The "c" configuration indicates a universal wing that was capable of accepting all types of allied guns plus the ability to carry two 250 lb bombs. The "e" designation indicates that the aircraft was armed with two 20 mm Hispano cannons, two 0.50-caliber Browning machine-guns and one 1,000 lb bomb.