Definition
Referral traffic is visits that come from links on other sites, identified by the referrer the browser sends. Search engines, social networks and AI assistants are usually grouped into their own channels, so "referral" covers the rest: blogs, directories, partners, news sites.
When the referrer goes missing
Browsers don't always send a referrer: links opened from apps, links set to noreferrer, and some privacy settings. Those visits arrive as Direct. Many clicks from ChatGPT arrive this way; see ChatGPT traffic in GA4.
Measuring its value
Group referrers by domain and compare the sign-ups and retained customers each brings. A partner sending few but well-matched visitors can be worth more than a large directory.