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

    Dismounts a volume throughout a dual-architecture OpenVMS
    Cluster system. If you specify DISMOUNT/CLUSTER, the DISMOUNT
    command checks for open files or other conditions that will
    prevent a Files-11 volume on the local node from dismounting.
    If the DISMOUNT command does not find any open files or other
    conditions, it checks for conditions on all other nodes in
    the OpenVMS Cluster. If the DISMOUNT command finds one of the
    conditions on any node, it displays an error message identifying
    the device and the nodes on which the error occurred, followed by
    an error message indicating open files or other conditions on the
    volume.

    After the DISMOUNT command successfully dismounts the volume on
    the local node, it dismounts the volume on every other node in
    the existing OpenVMS Cluster environment. If the system is not a
    member of a cluster, the /CLUSTER qualifier has no effect.