As Sam said above, each cashback site will input their own unique variable in to the affiliate network tracking link - this variable is unique to the user account, and is reported back to the publisher.
Many networks have different terminology (SID, EPI, Clickref etc) but all are essentially the same.
I happen to have a set of examples to hand (these are from Quidco tracking URLs) - the variables in blue are the unique reference.
Code:
http://valuemobiles.at/quidco?LID=2x90894842
http://clk.tradedoubler.com/click?epi=2x90894957&p=2554&a=1089147&g=17981984
http://www.awin1.com/cread.php?clickref=2x90895018&awinmid=2533&awinaffid=54760
http://www.tkqlhce.com/click-1699685-5590799?SID=2x90895069
http://being.successfultogether.co.uk/click.asp?subid=2x90895111&ref=394365&site=3982&type=text&tnb=3
http://scripts.affiliatefuture.com/AFClick.asp?tracking=2x90895161&affiliateID=23992&merchantID=4668&programmeID=12062&mediaID=0&url=
http://click.linksynergy.com/fs-bin/click?u1=2x90895201&id=3w57gQxcGGY&offerid=187513.10000005&subid=0&type=4
http://track.webgains.com/click.html?clickref=2x90895389&wgcampaignid=3920&wgprogramid=3083
As a side note - I would recommend all affiliate utilise this functionality, regardless of whether or not they are a cashback site. Suggested uses could be for tracking paid search activity (optimising keywords) or tracking which adverts/placements perform best on your site. Or at the very least, if you offer any form of promo codes on your site, inserting that so you can see how well they convert.
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