In a previous thread (see <http://forum.httrack.com/readmsg/14233/index.html>)
this topic was asked for and my answer, sadly, was wrong, or at least not good
enough... As soon as I realized, I created a testing website to check the
correctness of my answer. It is now online for you to check it out if you
wish: <http://www.jargoon.arrakis.es/test/depths/home.html>

It has been made to test what is a depth level, and what is has to do with
"clicks".

I've found there are some strange behaviours, as could be considering a link
to home page an out of reach page or not downloading/showing the background
image, which is the same for all pages.

In spite of that, I've found depth levels are relative levels to starting
pages, i.e. if a page is 5 links away from the starting page (the one you
specify when creating the project), it is 5 clicks away and in level/depth 6.

Anyway I still can't understand why a page in the last level specified
(--depth=6) that has a link to a previous one (e.g. a page in level 4 or even
the starting page) is redirected to an error/external page. Nor I understand
why a background image in same circumstances is not shown.

Test made with version 3.40
Hope this helps a bit