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

    VT100: Line Feed + Enter        VT200: Do + Enter
    DEFINE_KEY Function: ENTER

    Sends your input to the remote application program. While this
    communication is active, the keyboard locks and Inhib appears
    on the status line. Usually the application program releases the
    keyboard when it has finished processing your input.