A subdomain is always attached to your primary domain. For example, if your primary domain is' .example.com', then 'blog.example.com' or 'shop.example.com' are the subdomains. Whereas a completely different primary domain will be considered as cross-domain, such as 'xyz.com'.  To know more about cross-domain tracking, visit:  Cross-Domain Tracking in Google Analytics – Complete Guide Cross-Domain Tracking in GA4 (Google Analytics 4) Setup Guide