No idea what that is.
My only quest on this theory would be say you have
<Page somemainwindow>
</page>
Could include one of the other windows inside a page inside this one...
<page somemainwindow>
<page size="100,40">
</page>
</page
And then move that page around so that it would only show part of another window. So, like, resize the page, and display a distinct area of a main window... say the inventory bag area or something (just randomly off the top of my head). I'm guessing yes, but would the bags be open'able?