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

    VT100: Ctrl/Z or F10        VT200: Ctrl/Z or F10
    DEFINE_KEY Function: EXIT

    Terminates the remote TELNET/TN3270 session. Aborts any exchange
    of data in progress between the local and remote hosts. Note that
    terminating a session with the IBM host in this way may result in
    improper termination of the session. For the appropriate logoff
    command string, see the user's guide for the IBM application with
    which you are communicating.