I guess that's the session id. It stays the same and it's added to all links as long as you stay at Amazon.
Since this does not depend on cookies, it's more safe since cookies have to be turned on to work, this works with all browsers and can't be deactivated.
I think PHP's session support has a function that adds session IDs to URLs, but at least there is a variable containing the session ID.
I just read that if you compile PHP with "--enable-trans-sid" links are automatically changed to contain the session id. Pretty cool...
Matthias
