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

    VT100: EXT + A              VT200: F19
    DEFINE_KEY Function: ATTENTION

    Provides a way to "get the attention of" the remote application
    program that you are running by sending a SIGNAL RU command
    to the remote host. See the user's guide of the particular
    application program to learn what response the program gives
    when you use this key.