Identifiers & routing
What is Cross-domain tracking?
Stitching a user’s session across multiple domains (e.g. a site and a separate checkout/booking domain) so they’re counted as one journey, not two visitors.
Definition
Cross-domain tracking links a single user’s activity across more than one domain — a marketing site on one domain and a checkout, booking, or app subdomain on another — so analytics treats them as one continuous session instead of two unrelated visits.
It works by passing the client ID between domains (in GA4, via the linker parameter), so the destination domain recognizes the arriving user as the same one who left the first domain.
Why it matters for agencies
Many agency clients send users off-domain to convert — a Shopify checkout on `checkout.shopify.com`, an OpenTable booking, a third-party form. Without cross-domain tracking, the conversion looks like it came from a "referral" from the client’s own site, and attribution collapses.
It’s a setup that’s easy to forget at onboarding and invisible until someone notices the client’s own domain is their top "referral source" — a classic tell.
Cross-domain tracking in Phloz
Phloz’s GA4 property node captures the cross-domain linked-domains configuration, so an agency can document and verify which domains are stitched together rather than discovering the gap in a report.
Frequently asked questions
- How do I know cross-domain tracking is broken?
- The giveaway is the client’s own domain showing up as a top referral source, plus sessions that split in two at the domain boundary. Self-referrals in GA4 are the canonical symptom.
- Does cross-domain tracking work with iframes?
- Iframed checkouts and forms are a special case — the linker can’t always pass the ID into an iframe automatically, so they often need explicit configuration or a server-side approach. It’s one of the trickier setups to verify.
Related terms
Other tracking-infrastructure concepts agencies run into alongside this one.
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