They are short for WCHR, WCHS etc:
WCHR: Passenger who can walk up and down stairs and move about in an aircraft cabin, but requires a wheelchair or other means for movements between the aircraft and the terminal, in the terminal and between arrival and departure points on the city side of the terminal.
WCHS: Passenger who cannot walk up or down stairs, but who can move about in an aircraft cabin and requires a wheelchair to move between the aircraft, in the terminal and between arrival and departure points on the city side of the terminal.
WCHP: Passenger with a disability of the lower limbs who has sufficient personal autonomy to take care of him/herself but who requires assistance to embark or disembark and who can move about in an aircraft cabin only with the help of an onboard wheelchair.
WCHC: Passenger who is completely immobile who can move about only with the help of a wheelchair or any other means and who requires assistance at all times from arrival at the airport to seating in the aircraft, or if necessary, in a special seat fitted to his/her specific needs the process being inverted at arrival.