I found http://tv.adobe.com/watch/learn-flash-professional-cs5/xml-based-fla- files/
The XFL file structure is an "unzipped" FLA file http://www.leebrimelow.com/the-xfl-file-format-explained/ and very closely resembles the Encore output for web.
FYI: the FLA and SWF formats are Adobe proprietary while XFL is open source XML based.
See http://en.wikipedia.org/wiki/SWF for all Flash related formats.