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

    The DISMOUNT command closes all connections that EXCHANGE
    maintains to the device. This command does not affect the state
    of the operating system mount; the device remains accessible
    to OpenVMS. If you do not use the DISMOUNT command, an implicit
    DISMOUNT is automatically executed when you exit EXCHANGE.

    The DISMOUNT command is valid only with foreign devices.