Hi Claudius,
They are both at the end of the page after 99.9% of the HTML before the ending body tag, but they are separated by some JavaScript. The Page Hitbox content comes first, right after the footer and closing lia-page DIV, and followed by a bunch of Lithium JavaScript code (100+ lines in our pages). The content in the last_chance_html is literally at the very end because it is placed immediately before the closing </body> tag. In our case, we put our script tags in the Page Hitbox so they load after the HTML as is usually recommended. As for last_chance_html, I also use it for the one hoverCard.js file for Lithium's Profile Hover Cards which recommended putting it there, but I'm sure you could use it for other things. So in our case the page looks like this:
<html><head>[...typical header stuff...]</head>
<body>
<div class="lia-page">
<center>
[...all page content including header and footer...]
</center>
</div>
[...Page Hitbox content goes here...]
[...Next, lots and lots of Lithium JavaScript code...]
[...last_chance_html content goes here...]
</body>
</html>
Hope that makes sense.
Robert