In short. GA4 custom channel groups apply retroactively to reports, while primary channel group selection changes classification from that point forward.
Definition: Yes. As of 17 October 2026, Google Analytics documents that custom channel groups, rule-based categories of traffic sources, apply retroactively to reports. Selecting a custom group as the primary channel group changes that dimension going forward.
Key facts
- Google's custom channel groups documentation permits retroactive reporting with custom groups and applies audience eligibility changes going forward.
- Google's scope documentation assigns session-scoped acquisition dimensions new values when users return.
- Google's retention documentation distinguishes standard aggregated reports from explorations and funnel reports.
What changes when a group is edited?
Google's custom channel groups documentation says edited definitions immediately apply to underlying data in reports, explorations and segments.
The primary channel group uses the selected custom group's criteria from the point of selection onward.
Which acquisition scope describes historical visits?
A dimension is a reporting attribute. Google's scope documentation separates acquisition dimensions by user, session and event scope. Session-scoped dimensions describe where new and returning users come from when they start sessions. A session is a period of interaction with a website or app.
First user dimensions describe how new users originally arrive; their assigned values remain the same when users return. The User acquisition report presents this scope. The Traffic acquisition report presents session scope, which corresponds to visits.
Where can the historical classification be viewed?
Google's Traffic acquisition report instructions give this report path:
Reports > Acquisition > Traffic acquisition
That report includes Session source / medium, the source and medium associated with a new session, and Sessions, the number of sessions that begin on the website or app. It covers returning users as well as new users.
Google's custom group instructions allow the group as a primary or secondary dimension in acquisition reports. They also make custom groups available in custom reports and explorations. Custom groups are unavailable in the Key events paths report.
How far back can an exploration reach?
Google's data retention documentation says retention settings affect explorations and funnel reports. A date range extending beyond retention leaves the additional period unavailable in an exploration.
Standard aggregated reports, including primary and secondary dimensions, are unaffected by this retention setting, including when comparisons are added. Increasing retention applies to already collected data that has yet to be deleted.
Related terms
- GA4 traffic acquisition
- How do I create an AI assistants channel in GA4?
- How do I make a custom channel group the primary group in GA4?
- Why is older AI referral data missing from GA4 Explorations?
FAQ
Must I make the group primary to use it in reports?
No. Google's custom group documentation permits custom groups as report dimensions independently of selecting the property's primary group.
Why can acquisition reports describe different sources?
Google's scope documentation places first-user acquisition in User acquisition and session acquisition in Traffic acquisition. Returning users can start sessions with different acquisition values.
Sources
- Google Analytics Help, "Custom channel groups", undated
- Google Analytics Help, "Scopes of traffic-source dimensions", undated
- Google Analytics Help, "[GA4] Traffic acquisition report", undated
- Google Analytics Help, "Data retention", undated
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