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

       /OVERSTRIKE (default)

    Sets the terminal to overstrike mode. This feature allows you to
    type over the current character when you are editing a command
    line. Set your terminal to insert mode if you want to insert
    characters when editing command lines. Press Ctrl/A to switch
    from one mode to the other.