Session source dimension
The session source dimension attributes a whole session to a single origin — google, a newsletter, a domain — fixed when the session begins. GA4 derives it from the campaign tags or referrer present at session start. It differs from the event-scoped source (which can vary within a session) and from first-user source (the very first acquisition), so mixing the three is a frequent attribution error.
What this means
Session source is the origin GA4 assigns to an entire session, locked in when the session starts (a new session begins after a gap of inactivity or a new campaign). It answers 'where did this session come from?' as a single value for the whole visit.
This is a different scope from the source attached to individual events, which can reflect mid-session campaign changes, and different again from first-user source.
Scope pitfalls
First-user source records how the user was first acquired and never changes; session source can differ on every return visit. Event-scoped source can shift within a single session if a new campaign arrives. Comparing totals across these scopes, or summing them, produces attribution that does not reconcile. Pick one scope per question.
- Fixed at session start
- Differs from first-user (first-touch) source
- Differs from event-scoped source within a session
How it appears in analytics and logs
A session source value is the origin captured at session start. If it disagrees with first-user source, that is expected — the user was acquired earlier and returned via a different origin.
Diagnostic use case
Use session source for session-level acquisition reporting, keeping it distinct from first-user source and from event-scoped source within the same session.
What WebmasterID can help detect
WebmasterID fixes a session's origin from referrer + utm_* at session start and keeps it separate from first-touch, so session-level and acquisition reporting do not blur together.
Common mistakes
- Comparing session source with first-user source as if equal.
- Summing source across different scopes.
- Assuming session source updates mid-session.
Privacy and accuracy notes
Session source derives from referrer and campaign tags at session start, not from cross-site identity. WebmasterID attributes it first-party without fingerprinting.
Related pages
- First user source dimension
The first user source dimension records the origin of a user's very first session — their acquisition source — and keeps it fixed for the user's lifetime. GA4 sets it from the referrer or campaign on the first visit. It is user-scoped, so it answers 'how did we acquire this person?' rather than 'where did this visit come from?', and confusing it with session source distorts attribution.
- Source / medium: the core traffic-origin dimension
Source/medium is the dimension that records where a visit came from (the source, e.g. google) and how it arrived (the medium, e.g. organic). It is derived from the referrer and UTM parameters, with rules that vary by tool. The big caveat: when neither is available, the visit lands in 'direct / (none)', which is a catch-all, not a channel.
- Campaign vs referrer precedence
A single visit can arrive with both a Referer header and UTM campaign parameters. Most analytics tools let the explicit campaign parameters take precedence over the inferred referrer, because a deliberate utm_source is a stronger signal than a host guess. Knowing the precedence order explains why a tagged link shows your campaign source even when the referrer differs.
- Attribution analytics
Session vs first-touch attribution, kept distinct.
Sources and verification notes
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.