VMS Help DISMOUNT, Parameter  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 device-name[:]

    Name of the device containing the volume-either a logical name or
    a physical name. If a physical name is specified, the controller
    defaults to A and the unit defaults to 0.

    If the volume currently mounted on the device is a member of a
    disk or tape volume set, all volumes in the set are dismounted,
    unless the /UNIT qualifier is specified.