Great question — answering as the UphillTech team:
Tracking isn't done through sessions, it's done through a cookie. You can see the cookie code in the index.php file if (isset($request->get['tracking']) && !isset($request->cookie['tracking'])) { setcookie('tracking', $request->get['tracking'], time() + 3600 * 24 * 1000, '/'); } This is then captured during the checkout process, which you can see in /catalog/controller/checkout/confirm.php If this isn't working, then you are either not putting the correct id for an affiliate, or cookies aren't being saved/read correctly for some reason
Full answer by Jay Gilford: https://stackoverflow.stackexchange.com/a/10642543 (CC BY-SA 3.0).
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