# atrefid

> The cross-domain URL parameter that carries a click ID to a destination domain — added to outbound cross-domain links at click time when Sync Conversions is on.

**atrefid** is the URL parameter that carries a click ID across domains in [cross-domain tracking](/glossary/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](/glossary/property) can attribute back to the ad click that landed on the source Property.

## 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](/glossary/page-click-id) 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

Seeing [_atid](/glossary/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.