In short. GA4 Traffic acquisition groups sessions by source and medium, with recognized assistant referrers assigned the ai-assistant medium.

Definition: GA4 traffic acquisition is session-level reporting in Google Analytics 4. As of 17 October 2026, its Traffic acquisition report describes where new and returning users start sessions, using fields such as Session source and Session medium.

Key facts

Google's Traffic acquisition report documentation defines the Session source / medium field as the source and medium associated with a new session.
Google's scope documentation assigns new session-scoped values when users return to a website or app; First user values remain attached to the user's original acquisition.
Google's default channel definitions include an AI Assistant channel for sources such as ChatGPT and Gemini, with the medium ai-assistant.

What do session source and medium describe?

A session is a period during which a user interacts with a website or app. A dimension is a descriptive field used to group report data. In Google's Traffic acquisition report, Session source identifies the publisher or inventory source from which traffic originates. Session medium describes the method of acquiring users in a new session. Session source / medium combines those fields in the report.

For assistant traffic, reading both fields preserves the distinction between the originating service and the method assigned to the visit. The Sessions metric counts sessions that begin on the website or app. The report also includes Key events, which counts how often users trigger events marked as important to the business, and Session key event rate, the percentage of sessions containing a key event.

How does Google classify assistant referrals?

A channel group categorizes traffic through rules. Google's Default channel group documentation defines AI Assistant using the medium ai-assistant. When the referrer matches Google's list of AI assistants, the documentation specifies these values:

medium: ai-assistant
campaign: (ai-assistant)

The same documentation names ChatGPT, Gemini, Deepseek, Copilot and Grok as examples of sources in this channel. It places visits from non-ad links in Google's AI Overviews and AI Mode within Organic Search.

The distinction affects report filters: an assistant traffic review needs to inspect ai-assistant, alongside any relevant recorded sources. A filter restricted to referral selects a different medium. Google's Referral channel rule accepts the medium values referral, app or link.

How is the report inspected?

Google's report instructions give the navigation path Reports > Acquisition > Traffic acquisition. Selecting Session source / medium as the table dimension exposes the source and method together. The search bar above the table finds individual traffic sources; Google's example searches for google / organic.

For assistant analysis, the same approach uses the source and medium recorded in the property's table. Sessions and Session key event rate then describe visit volume and the share of sessions containing a marked key event. For a particular page, the documentation provides a report filter using Page path and screen class, or the option to add that field as a secondary dimension.

FAQ

Should I use First user source for returning assistant visitors?

Use Session source when the question concerns the origin of a session. Google's scope documentation describes First user source as the source that originally acquired the user; its value persists across return visits.

Does changing the attribution model change session source?

Google's same scope documentation states that user-scoped and session-scoped dimensions are unaffected by changes to the attribution model. Event-scoped dimensions, such as Source without a prefix, use the selected attribution model to assign credit for key events.

What if I cannot find Traffic acquisition?

Google's report documentation says the report may have been removed or may be absent from the default view. An Editor or a user with higher permissions can add it back to the navigation.

Sources

Check how a page reads to answer engines. Open the GEO score analyzer