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

    VT100: KP.                  VT200: KP.
    DEFINE_KEY Function: EXTEND

    Used in conjunction with another key, allows access to an
    extended function for that key. First invoke the EXT function
    and then press the second key. If you invoke EXT accidentally,
    invoking the RESET function cancels the EXT function.

    If the status display is enabled when you invoke the EXT
    function, the word Extend appears on the status line.