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

    WINDOW_SYSTEM specifies the windowing system to be used on a
    workstation. Specify one of the following values:

    Value  Description

    1      Load the DECwindows Motif for OpenVMS workstation
           environment.
    2      Load the UIS workstation environment.

    WINDOW_SYSTEM is a DYNAMIC parameter.