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

       /ASSIST (default)
       /NOASSIST

    Command Qualifier

    Allows operator or user intervention during a BACKUP operation if
    a magnetic tape mount request fails or if an operation requires
    another volume. The /ASSIST qualifier causes BACKUP to send
    messages to operator terminals when a failure occurs during
    a BACKUP mount request or when an operation requires another
    volume. The default is /ASSIST.

    If you specify /NOASSIST, mount messages appear on your terminal
    and are not sent to the operator. The /NOASSIST qualifier has no
    effect if the logical name SYS$COMMAND points to a device that
    is not a terminal (as is the case when you run BACKUP in a batch
    job).