VMS Help INITIALIZE, 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 any device that supports data compaction.
    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.