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

    VT100: Tab                  VT200: Tab
    DEFINE_KEY Function: TAB

    Moves the cursor to the first character location of the next
    unprotected field on your screen. If the screen has no fields,
    the Right arrow | function moves the cursor to the first location
    on the screen.

    If the cursor is within the last unprotected field on the screen,
    the cursor moves to the first position of the first unprotected
    field on the screen.