# 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 [atclid](/glossary/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

With Sync Conversions ON, `atrefid` is a round trip, not a one-way handoff:

1. The visitor clicks the cross-domain link. The **source** Property tracks the `OutboundClick` event.
2. `atrefid` and its value pass to the destination domain.
3. The destination page carries the Tracking Tag, which reads `atrefid` from the page URL and stores the value.
4. A conversion recorded on the destination and tied to that `atrefid` value is sent **back to the source Property**.
5. 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

Seeing [_atid](/glossary/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.