A channel grouping is a set of rules that assigns each session's traffic source to a channel, such as Organic Search, Direct, Referral, Email or Paid Social. GA4 applies a Default channel group to every property and lets you create custom channel groups with your own rules. Channels make reports readable, but they are only as accurate as the source and medium values your links send.
How the default rules work
GA4 assigns channels by matching source, medium and campaign values against published rules. For example, Organic Search matches when the source is on Google's list of search sites or the medium is exactly organic. Paid channels depend on medium values such as cpc or paid. Anything that matches no rule lands in Unassigned.
Google maintains the rule definitions and source lists in its help center and updates them from time to time, so check them when a channel suddenly changes.
Why UTMs matter
Most misclassified traffic comes from inconsistent UTM parameters. A newsletter tagged with utm_medium=newsletter instead of email may land in Unassigned or another bucket. Agree on a short list of medium values and stick to it.
- email for all email campaigns
- cpc or paid_social for paid ads, matching the default rules
- referral only for genuine third-party links
Custom channel groups
Custom channel groups let you add channels the defaults lack, such as AI assistants (by matching known AI referrer sources), partner marketplaces or a review site. They apply to past data in reports, which makes them a safe way to experiment.
Example
Giving AI referrals their own channel
Suppose Vellumspire, a knowledge base SaaS, notices visits from chat assistant domains scattered across Referral. It creates a custom channel group with an AI assistants channel matching those sources. The team can now track that traffic's trend and key events on its own line, while noting that many AI-driven visits still arrive without a referrer.
Common mistakes
Inventing new medium values freely
Every new medium is a chance to fall outside the default rules. Keep the list short and documented.
Editing the default instead of copying it
Build custom groups as copies so the default remains for comparison.
Questions
The session's source, medium or campaign did not match any channel rule. It usually points to inconsistent UTM tagging.
