# atclid

> The URL parameter that carries a Click ID value on cross-domain links when Sync Conversions is off, on email links back to your site, and on links into a SamCart checkout.

**atclid** is a URL query parameter. Its value is an AnyTrack [Click ID](/glossary/click-id). The Click ID is the generated value; `atclid` is one of the names that value travels under, in the same way as [_atid](/glossary/atid) and [atrefid](/glossary/atrefid).

## Where atclid is used

- **Cross-domain links, when Sync Conversions is OFF.** Links pointing at another domain you own carry `atclid`. When Sync Conversions is ON, they carry [atrefid](/glossary/atrefid) instead, and the conversion recorded on the destination returns to the source Property.
- **Email marketing.** If you collected the Click ID value at form submission and stored it against the subscriber, append it to the links in your emails: `propertyURL.foo/?atclid=<value>`. When the subscriber lands on a page where the Tracking Tag loads, AnyTrack reads the value and resolves the visitor's identity from earlier sessions and conversions, so later conversions attribute back to the original ad click.
- **SamCart.** Links from your landing page into the SamCart checkout are autotagged with `atclid=<value>`, so the completed order carries the Click ID back to AnyTrack.

## What atclid is not

`atclid` is not the identifier itself — see [Page Click ID](/glossary/page-click-id) for the 30-character page-session value and [Click ID](/glossary/click-id) for the 34-character element value. It is also not the parameter for same-page click-ID collection: that is [_atid](/glossary/atid), which the Tracking Tag appends to the current page URL.