A quick draft is available for the new httrack cache format 
(new.zip) at this location:
<http://www.httrack.com/html/cache.html>

This new format will be used by upcoming httrack versions. 
The current 3.31-alpha-4 release is yet using it.
(And again, the future releases of httrack will still be 
able to import previous cache formats, for updating purpose)