Who sets awc, and when
awc is written into the URL by Awin's own redirect domain the moment a shopper clicks a publisher's tracking link, before the browser reaches the advertiser's site. Neither the publisher nor the advertiser generates the value. Awin's servers do, as part of forwarding the click.
The structure, and the publisher/advertiser split
A real awc value is typically written as three parts joined by underscores, something like 3661_1358445475_f396d719f0b67247f126a90bd2898cf0. That shape matters because Awin is a two-sided network. Publishers place the tracking links and earn the commission. Advertisers are the merchants whose sites receive the traffic and pay it. awc is what lets Awin, after the sale, work backward from a transaction on the advertiser's site to the exact publisher whose link produced the click, so it knows who to pay.
The parameter people mix it up with
Awin also has a second, separate parameter called clickref, which a publisher can set to label their own traffic sources for their own reporting. It rides alongside awc but plays no part in calculating commission. Mixing the two up is the most common reason people search for what awc actually does. They see clickref, assume it is the important one, and strip or ignore awc instead.
What Awin can no longer do without it
A visitor is unaffected. Checkout works the same. For Awin, though, awc is the only thing connecting the sale back to the click. Strip it and the network cannot validate that connection, so the publisher's commission is not paid, and Awin's fraud and duplicate-click checks, which lean on this same token, cannot run either.
Treat awc as case-sensitive. Copy it exactly. For the other two affiliate click IDs in this family, see irclickid and cjevent, or drop your full URL into the link decoder to see awc split out from the rest of the query string.