Master

    Controls the assignment of tape drives to output threads by
    allowing you to specify a tape drive as a master tape drive. This
    is a positional qualifier specified with a tape drive. When the
    Master qualifier is used, it must be used on the first tape drive
    specified. When the Master qualifier is specified, all additional
    tape drives become slaves for that tape drive until the end of
    the command line, or until the next Master qualifier, whichever
    comes first.

    If you specify the Master qualifier (without also specifying the
    Loader_Synchronization qualifier) on sets of tape drives, each
    master/slave set of tape drives will operate independently of
    other master/slave sets. If the Master qualifier is used on a
    tape drive that is not physically a master tape drive, the output
    performance of the backup operation will decrease.

    See the Oracle Rdb Guide to Database Maintenance for complete
    details on the behavior of the master qualifier.