Juts to think about the difference a bit more, in MDI you could open new windows within the application-window, say multiple text-editor-windows within the MS W

Juts to think about the difference a bit more, in MDI you could open new windows within the application-window, say multiple text-editor-windows within the MS Word application-window, if I recall. What you could not do is open a new application-window from within the application window. And that would seem rather useless. But it would not be useless if the whole desktop worked that way, open new child-desktops from current one, recursively.