Of the three functions for opening a window, only openwindow() is present
in all versions of the OS.  This function takes a newwindow structure as
its sole argument.  In order to allow applications to use the OpenWindow()
call with Release 2 tagitem attributes, an extended version of the
NewWindow structure has been created named extnewwindow.

Setting WFLG_NW_EXTENDED in the NewWindow.Flags field specifies to the
openwindow() call that this newwindow structure is really an extnewwindow
structure.  This is simply a standard NewWindow structure with a pointer
to a tag list at the end.  Since WFLG_NW_EXTENDED is ignored prior to V36,
information provided in the tag list will be ignored by earlier versions
of Intuition.  Note that WFLG_NW_EXTENDED may not be specified in the
wa_flags tag.