Why fixing 'Direct/None' alone won't reduce GA4's 'unknown traffic'

A friend running a D2C brand told me last week: "I enforced UTMs across every ad and email link to reduce Direct / (none) in GA4. The Direct share dropped, but the total 'unknown traffic' in the report barely changed. What did I miss?" I dug into it. The answer is straightforward — and slightly embarrassing for me, because I had never internalized it cleanly either. The "unknown traffic" you see in GA4 is not one category. It's four. Direct / (none), (not set), Unassigned, and (other). Each is g