/sys$common/syshlp/helplib.hlb SET, TERMINAL, Qualifiers, /INSERT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Sets the terminal to insert mode. This feature allows you to
    insert characters when editing command lines. The default mode is
    overstrike, which allows you to type over the current character
    when editing a command line. Press Ctrl/A to switch from one mode
    to the other.