Restrictions

Only one subsystem can claim ownership of a command and assume responsibility for its processing by assigning a unique command prefix to the subsystem; any command prefixed by that command prefix is owned by that subsystem.

Note:

A command prefix is a character string of one or more alphanumeric and/or national characters. Command prefixes often have a length of one character, although a maximum of eight characters is permitted.
See the CPF macro in z/OS MVS Programming: Authorized Assembler Services Reference ALE-DYN for information on registering command prefixes.