VMS Help BACKUP Command, Qualifiers, /INCREMENTAL  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Command Qualifier

    Allows you to restore a disk volume from a series of incremental
    save sets. When processed correctly, the output disk volume
    will contain the same files it contained when the most recent
    incremental save operation was performed. BACKUP does not create
    files on the output disk volume that were deleted in the period
    in which the incremental backups were performed.

    The /INCREMENTAL qualifier is valid only in restore operations.
    You must specify a device as the output specifier; file
    specifications are invalid. When you specify the /INCREMENTAL
    qualifier, BACKUP restores files using the /BY_OWNER=ORIGINAL
    qualifier.