P is
normally a positioning flight, F a ferry flight, M a maintenance flight, but it depends on the operator.
Because of the growth in flights, and the move away from aircraft registration call-signs to flight numbers, there will often be two or more similar numbers in the system at a given airport at the same time. This has given rise to airlines using a wide range of different formats, such as Wizzair 201G (nnnl), Easy 3QH (nll), Monarch 846 (nnn), Midland 7985 (nnnn). At one time, Britannia used call-signs such as BY123A for a UK outbound leg and BY123B for the inbound. Likewise, Thomas Cook used K and L suffixes. Some airlines always use the same letter (Titan seem to like Y), others vary it according to destination/origin.
In other words, there are no rules!!
