Message numbers ARC9000 through ARC9299 have been set aside for use by DFSMShsm installation exits, either as samples supplied by DFSMShsm or messages written by customers. For an explanation of these messages, customers have to locate the issuing exit or any user-created documentation for that exit.

Message numbers ARC9000 through ARC9199 are intended for use specifically for the ARCRPEXT exit. The ARCRPEXT installation exit can pass messages back to DFSMShsm, and DFSMShsm writes these messages in the migration or backup activity log. User messages issued by other DFSMShsm installation exits are generally write-to-operator (WTO) messages.