Technical
Cross-domain tracking
Also called linker.
Cross-domain tracking is passing a visitor identifier from one site you own to another, usually in the URL, so both are recorded as one visit. Without it the second domain sees a stranger arriving from a referrer.
How it is measured
Click a link from site A to site B and read the address bar. A linker parameter such as `_gl=1*abc123` has been appended. B's script reads it, adopts the client id, and writes its own first-party cookie. In the network tab, confirm the id sent to the collector matches on both hosts.
A setting lists which domains to decorate. Links to unlisted hosts stay plain, and a form POST or redirect that drops the query string loses the id.
Worked example
A ticket seller runs `tickets.livewire.example` for browsing and `pay.livewire-checkout.example` for payment. Before linking, 2,100 purchases are attributed to a referral from the browsing site. After decorating links, 1,980 of them carry their original campaign.
The other 120 went through a wallet redirect that strips the query string. Those remain referrals until the redirect keeps the parameter.
How it differs
Cross-domain tracking carries identity across the border; a session is the window of activity that identity groups. Linking does not lengthen a session. It only stops a new one starting when the visitor crosses to your other host.
Common errors
Linking subdomains that already share a cookie. Forgetting to list the payment domain. Using redirects that drop the query. Trying to pass the id through an iframe by URL. Leaving the payment host in the referral exclusion list so it steals credit. Decorating links to domains you do not own.
In practice
Click through the real path once per release with the network tab open and compare the id at every hop. Add each host in the journey, checkout included, and exclude your own domains as referral sources. If both sites can live under one registrable domain, do that instead.