It is not required that all reservation types be used:
An "A" reservation is the standard full commission reservation. Someone calls, wants to stay in a unit and you book them a reservation.
A "B" reservation is an owner-referred reservation. An owner calls and has secured someone to stay in his unit. The guest is charged the rental amount plus any taxes and, typically, a lower commission is charged the owner since he secured the guest, although it could be higher or the same. The commission percentage charged for this type of reservation is the "B" commission set up in the control file or the unit file.
The standard "C" reservation is when an owner is staying in his own unit and requests you perform a cleaning when he leaves.
A "CY" is used for when a friend or guest of the owner is staying in the unit. The owner is charged the cleaning fee and a coordination fee.
A "D" reservation is used when an owner is staying in his unit and is going to clean the unit himself.
An "E" reservation is a long-term rental.