/sys$common/syshlp/helplib.hlb BACKUP Command, Qualifiers, /MEDIA_FORMAT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /MEDIA_FORMAT=[NO]COMPACTION

    Output Save-Set Qualifier

    Controls whether data records are automatically compacted and
    blocked together. Data compaction and record blocking increase
    the amount of data that can be stored on a single tape cartridge.

    The compaction ratio depends on the data and the tape drive you
    use. For more information, refer to the documentation supplied
    with your tape drive.

    BACKUP allows you to specify different compaction settings on
    different save sets on a tape. However, not all tape drives
    support the use of more than one compaction setting on a tape.
    Whether mixed mode tapes are permitted depends on the model of
    the tape drive you use.