Should an AI acquisition report use session source or first user source?

Definition: An attribution scope is the level at which analytics assigns a traffic source: user, session or event. An AI acquisition report uses Session source to measure sessions attributed to AI referrals, and First user source to examine users whose initial acquisition comes from those sources.

Key facts

• As of 17 October 2026, Google's scope documentation assigns First user dimensions to initial acquisition and Session dimensions to new sessions from both new and returning users.
• Google's BigQuery attribution guide identifies session_traffic_source_last_click as the session attribution record, available on every event.
• Google's export schema defines collected_traffic_source as traffic-source data present within collected events.

Which question does each scope answer?

For a report asking how many sessions are attributed to AI sources, Session source is the appropriate dimension. A session is a period of interaction with a website or app. Google's Traffic acquisition report includes both new and returning users and offers Session source / medium alongside the Sessions metric.

First user source answers which source originally acquired a user. Google's scope documentation states that this value remains the same when the user returns. Filtering on an AI source at this scope therefore selects users by their original acquisition. Session dimensions receive values for each new session.

Which BigQuery fields correspond to these scopes?

Google's BigQuery attribution guide maps first-user acquisition to traffic_source and session attribution to session_traffic_source_last_click. These source and medium paths expose the distinction:

traffic_source.source
traffic_source.medium

session_traffic_source_last_click.manual_campaign.source
session_traffic_source_last_click.manual_campaign.medium

collected_traffic_source.manual_source
collected_traffic_source.manual_medium

Google's export schema describes the first pair as the network and medium that first acquired the user. It also provides traffic_source.name for the initial marketing campaign. The session record contains last-click attribution across advertising and manual contexts, where available; its cross_channel_campaign.source and cross_channel_campaign.medium fields describe the last clicked cross-channel campaign.

The collected record stores values collected with the event. Its manual source and medium fields include parsed referral parameters as well as UTM campaign parameters, which are URL labels such as utm_source and utm_medium. These fields describe collected inputs; the session record contains the processed session attribution. The schema also states that traffic_source is unpopulated in intraday tables.

How should the report be set up?

Google's Traffic acquisition instructions give this navigation path:

Reports > Acquisition > Traffic acquisition

For a session acquisition report, select Session source / medium and use the report's source search or a report filter to select the AI source values recorded in the property. The report defines Sessions as sessions that began on the website or app. It also provides Key events, the number of times users trigger an event marked as a key event, and Session key event rate, the percentage of sessions containing one.

A report organized around original acquisition uses First user source in User acquisition, as described in Google's scope documentation. The same documentation assigns user and session dimensions to paid and organic channels last-click attribution; changing the reporting attribution model leaves these scopes unchanged.

Related terms

• GA4 traffic acquisition
• AI referral traffic
• UTM parameters
• How do I query AI referral sessions in the GA4 BigQuery export?

FAQ

Which source should I use for returning visitors?

Use Session source to analyze their sessions by acquisition source. Google's scope documentation includes returning users in session dimensions and keeps First user values unchanged when users return.

Does the unprefixed Source dimension have the same scope?

Google's scope documentation assigns the unprefixed Source and Medium dimensions to event-level attribution for key events. Those dimensions follow the selected reporting attribution model, whose default is data-driven attribution.

Can I use traffic_source on a purchase row as session source?

Google's export schema defines traffic_source as first-user acquisition. Use the session attribution record for a session-scoped analysis, even when examining a purchase event.

Sources

1. Google Analytics, "Traffic attribution data", updated 14 April 2026
2. Google Analytics Help, "Scopes of traffic-source dimensions", accessed 17 October 2026
3. Google Analytics Help, "BigQuery Export schema", accessed 17 October 2026
4. Google Analytics Help, "[GA4] Traffic acquisition report", accessed 17 October 2026

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