VT100: EXT + R              VT200: EXT + F19
    DEFINE_KEY Function: SYS_REQUEST

    Lets you shift between the application program (the LU-LU
    session) and the control program (the SSCP-LU session). If the
    status display is enabled, the Appl or SSCP indicator appears
    on the status line to indicate the type of session. Appl appears
    when you are in an LU-LU session, and SSCP appears when you are
    in the SSCP-LU session.

    The screen is refreshed when you use the SYS REQ function.