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

    VT100: EXT + E              VT200: EXT + Find
    DEFINE_KEY Function: ATTACH

    Changes control from one subprocess to another subprocess or to
    the parent process. When you invoke the ATTACH function, TN3270
    uses the name of the last process to which you attached as the
    default process name.

    If you want to attach to a different process, press Ctrl/U to
    erase the default process name. You can then enter the process
    name of your choice at the prompt. The process name can be a
    quoted string. Use the quotation marks to preserve spaces, tabs,
    or lowercase letters in strings.