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 the 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.
What atrefid is not
Section titled “What atrefid is not”Seeing _atid on URLs is unrelated — _atid is platform-generated for click-ID collection on the same site and has nothing to do with cross-domain tracking.
Frequently asked questions
FAQ was last reviewed on 2026-08-01
I don't see atrefid in my page's HTML source. Is it missing?
No. atrefid is added to the link at click time (on click-through), so it never appears in fetched or served HTML. Test by clicking the link and reading the destination URL.
When do cross-domain links carry atclid instead of atrefid?
When Sync Conversions is OFF the links carry atclid; when Sync Conversions is ON (the standard cross-domain setup) they carry atrefid.