If the catalog entry, the data set VTOC entry, or both are missing, the data set is an unmatched data set. For an unmatched data set, the space management and backup exit (ARCSAEXT) is invoked, if the exit is enabled. If the exit indicates that processing continues, or if the exit is not enabled, DFSMShsm issues a message, does no further processing on the unmatched data set, and begins processing the next data set. If the exit indicates that processing is not to continue, DFSMShsm begins processing the next data set without issuing any message.