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

       /MEDIA_FORMAT=[NO]COMPACTION

    Controls whether data records are automatically compacted and
    blocked together on a TA90E tape drive. Data compaction and
    record blocking increase the amount of data that can be stored
    on a single tape cartridge.

    Note that once data compaction or noncompaction has been selected
    for a given cartridge, that same status applies to the entire
    cartridge.