Cross-domain tracking lets analytics treat shop.com and checkout.shop.com (or shop.com + payments.processor.com) as one continuous session. Without it, every domain crossing creates a new session and breaks attribution. GA4 handles this via linker config — gtag passes _gl URL parameters between domains, decoded on the destination side. Plausible has no built-in cross-domain stitching — you’d implement it yourself with a shared visitor_id custom property. Matomo has native cross-domain support via the JavaScript tracker. Migration consideration: if your funnel crosses domains, the privacy-first tool’s session model may differ from GA4’s.
Cross-domain tracking
Linking a single user session across two or more domains you control — essential for checkout flows on separate domains.