atrefid is the URL parameter that carries a click ID across domains in cross-domain tracking: a click ID value is generated and passed on links pointing at the destination domain, so a conversion recorded on the destination Property can attribute back to the ad click that landed on the source Property.
How atrefid behaves
Section titled “How atrefid behaves”- It is appended at click time (on click-through), so it does not appear in the page’s served HTML — verify it by clicking, not by viewing source.
- It is used when Sync Conversions is ON on the cross-domain setup; with Sync Conversions OFF, cross-domain links carry atclid instead.
- All cross-domain settings live on the source Property: the destination-domain list, the Sync Conversions toggle, and the synced event-names allowlist.
How atrefid returns the conversion to the source Property
Section titled “How atrefid returns the conversion to the source Property”With Sync Conversions ON, atrefid is a round trip, not a one-way handoff:
- The visitor clicks the cross-domain link. The source Property tracks the
OutboundClickevent. atrefidand its value pass to the destination domain.- The destination page carries the Tracking Tag, which reads
atrefidfrom the page URL and stores the value. - A conversion recorded on the destination and tied to that
atrefidvalue is sent back to the source Property. - The source Property tracks the conversion, attributes it to the original session and its UTM parameters, and sends it to the connected ad platforms through their Conversion API.
This return leg is why cross-domain attribution credits the campaign that produced the first click, even though the conversion happened on the other domain.
What atrefid is not
Section titled “What atrefid is not”Seeing _atid on URLs is unrelated — the Tracking Tag appends _atid to the current page URL for click-ID collection on the same site, and it has nothing to do with cross-domain tracking.
Frequently asked questions
FAQ was last reviewed on 2026-08-01