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

    Displays the window count and total size of all windows for files
    open on a volume. The file name and related process name and
    process identification (PID) are also displayed. The letter C
    in a display indicates that the file is open with "cathedral
    windows" (segmented windows).