Definition
A click ID is a unique identifier generated for each individual click on a tracking link, carried through the redirect into the offer URL, and stored by the advertiser so it can be sent back on the server-to-server postback when a conversion occurs. It is the join key that lets an affiliate's tracker match a conversion reported by the advertiser to the exact click — and therefore to the exact source, campaign, creative and SubID — that produced it.
Because the click ID travels as a URL parameter and is echoed back by the advertiser rather than read from a browser cookie, click-ID attribution is resilient to the things that break cookie tracking: third-party-cookie blocking, tracking-prevention features, cross-device journeys, and ad blockers. This is why server-to-server postback attribution keyed on a click ID has become the standard for performance affiliate tracking, with cookies relegated to a fallback.
In context
The mechanics are simple but error-prone. The affiliate's tracker appends a click-ID macro to the offer link (e.g. &cid={clickid}); the traffic source replaces the macro with the real value at click time; the advertiser stores that value against the session; and when the user converts, the advertiser fires a postback to the tracker's endpoint including the click ID and event details.
If any link in that chain is misconfigured — a missing or misspelled macro, the advertiser not storing the parameter, the postback not including it — conversions come back with no click to attach to and land in an "unknown" bucket or go uncounted.
Because of this, verifying the click-ID flow end to end is a standard pre-launch step: click the link, complete a test conversion, and confirm the postback arrives at the tracker with the correct click ID and matches it to the test click. Discrepancies between an affiliate's numbers and an advertiser's often trace to click-ID handling — a truncated parameter, a case-sensitivity mismatch, a value overwritten by an intermediate redirect — so it is one of the first things checked in a reconciliation dispute.
Worked example
An affiliate's tracker shows 30% fewer conversions than the advertiser's report. Testing the click-ID flow reveals an intermediate safe-page redirect is stripping the &cid parameter, so those conversions come back to the advertiser without a click ID and are never matched by the affiliate's tracker.
Fixing the redirect to preserve the parameter closes the gap.
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