An affiliate cookie is a small record a merchant stores in a visitor’s browser saying where they came from. It exists for one purpose: so a sale can be credited to the person who referred it.
Everything else people say about affiliate tracking follows from that one mechanism, so it is worth understanding properly.
What happens when someone clicks
Your link carries an identifier unique to you. The reader clicks, passes briefly through the network’s tracking domain, and lands on the merchant’s site. During that redirect, a cookie is written recording your identifier and a timestamp.
At checkout the merchant reads the cookie, sees your identifier, and reports the sale to the network as yours. The network credits your account, and after a validation period the money becomes payable.
Two things follow immediately. The cookie has to survive from click to purchase, and it has to still be the one being read at the moment of checkout.
What it holds, and what it does not
| In the cookie | Not in the cookie |
|---|---|
| Your affiliate identifier | The buyer’s name or email |
| The time of the click | Payment or card details |
| Sometimes a campaign or link label | Anything you can read as the affiliate |
| An expiry date | Browsing history from other sites |
Why so many are lost
They expire. Windows run from 24 hours to 90 days. Buy after it lapses and nobody is credited.
They get overwritten. Most programmes credit the last click, so a coupon site clicked at checkout replaces yours entirely.
Browsers block them. Safari and Firefox restrict third-party cookies by default. Private windows discard them at the end of the session.
Devices change. Click on a phone, buy on a laptop, and there is no cookie to read.
The combined effect is significant. A meaningful share of the sales you genuinely cause will never be credited to you, and that is a structural property of the system rather than a fault to be fixed.
What is replacing them
Third-party cookies have been degrading for years, so networks increasingly use server-to-server tracking, where the merchant reports the referral directly to the network rather than relying on the browser.
It is more reliable and invisible to you as a creator. If a programme offers it, that is a point in its favour — it usually means fewer lost sales.
What this means practically
Check the cookie window before the commission rate. Expect your tracked sales to understate your real influence. And do not spend long investigating a specific sale that did not credit — there are half a dozen ordinary explanations and you will rarely establish which one applied.