VMS Help SET, MAGTAPE, Qualifiers, /RETENSION  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Moves a TZK10 tape cartridge to the end of the tape, and then
    back to the beginning of the tape. Use the /RETENSION qualifier
    on a regular basis to help maintain the integrity of TZK10 tape
    cartridges.

    The /RETENSION qualifier must be used in conjunction with the
    /REWIND or /UNLOAD qualifier. The /RETENSION qualifier completes
    its action before /REWIND or /UNLOAD. Use /RETENSION/REWIND when
    you want the tape cartridge to remain loaded in the drive. Use
    /RETENSION/UNLOAD when you want to unload the tape cartridge
    after the retension operation.

    This qualifier only affects TZK10 tape cartridge drives and has
    no effect on non-SCSI drives.