/sys$common/syshlp/helplib.hlb SHOW, DISPLAY, Parameter  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 display-device

    Refers to the display-device parameter specified with the SET
    DISPLAY command. If you are directing application output to
    multiple workstations in the same session, you can use logical
    names to point to each workstation. Using the SHOW DISPLAY
    command, you can specify this logical name as the display-device
    parameter to see where application output will be displayed.

    If you do not specify a display-device string, the logical name
    DECW$DISPLAY is used.