VMS Help TCPIP Services, TELNET, TN3270 Keyboard Functions, SYS_REQ  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    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, Appl or SSCP 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.