utm_medium and utm_source. If your utm_medium doesn't match what GA4 expects — because of casing, typos, or a made-up value — the traffic falls into Unassigned or the wrong channel. Clean, consistent mediums are what keep the grouping correct.The Default channel group is the report most people actually look at: "how much traffic came from Paid Search vs Email vs Organic Social?" But GA4 doesn't read your mind — it applies a rulebook to your UTM parameters. Understand the rulebook and you understand why your channels sometimes look wrong.
For each session, GA4 checks utm_source, utm_medium, and (for some channels) the source category, and matches them against a set of definitions. A simplified view of the most common ones:
| Channel | Roughly matches when… |
|---|---|
| Direct | no source/medium (or (not set)) |
| Organic Search | medium = organic, or source is a known search engine |
| Paid Search | medium matches cpc / ppc / paid and source is a search engine |
| Paid Social | medium matches paid patterns and source is a known social network |
| Organic Social | medium = social / social-network / sm, source is social |
medium = email / e-mail / e_mail | |
| Referral | medium = referral (or a referring site with no UTM) |
| Unassigned | nothing matched — the catch-all |
The rules match on specific strings. So a medium that's almost right still misses:
Email or CPC may not match cleanly.utm_medium=newsletter, social-paid, or fb don't match any definition → Unassigned.paid social (with a space) or emial won't match paid-social / email.utm_source=facebook&utm_medium=cpc can land in Paid Search instead of Paid Social if the pairing confuses the rules.The channel grouping is only as good as the utm_medium you feed it. Garbage mediums in, "Unassigned" out — and a chunk of your traffic vanishes from the channel report entirely.
There are actually two separate issues, and it's worth keeping them straight:
utm_medium doesn't match GA4's rules (this article).Both come from the same root cause: inconsistent UTM values. Fix the inputs and both problems go away.
cpc, paid-social, organic-social, email, display, referral, affiliate — not invented ones like newsletter or fb.You can build a custom channel group in GA4 to catch some of your non-standard mediums, but that's a band-aid on top of messy inputs — it doesn't fix the underlying data, and you'll rebuild it every time someone invents a new value. Cleaner inputs beat cleverer grouping every time.
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