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

    Creates a user-mode logical name in the specified table.

    If you specify a user-mode logical name in the process logical
    name table, that logical name is used for the execution of
    a single image only; user-mode entries are deleted from the
    logical name table when any image executing in the process
    exits; that is, after any DCL command that executes an image or
    user program completes execution. Also, user-mode logical names
    are automatically deleted when invoking and exiting a command
    procedure.