Session default channel grouping
Session default channel grouping rolls a whole session's source/medium into one channel — Organic Search, Email, Referral — based on the origin captured at session start. It is the session-scoped sibling of the user-scoped 'first user' channel and the event-scoped channel. GA4 offers several channel dimensions at different scopes, and mixing them is a leading cause of attribution numbers that do not reconcile.
What this means
Session default channel grouping applies GA4's ordered channel rules to the source, medium, and campaign captured when a session starts, producing one channel for the entire session. It is the channel-level view of session source/medium.
GA4 deliberately exposes channel at several scopes — first user (user-scoped), session (session-scoped), and at the event level — because each answers a different attribution question.
Scope discipline
First-user channel never changes; session channel can differ every visit. A report that sums sessions by first-user channel and sessions by session channel will not match, and that is correct, not a bug. Choose the scope that fits the question — acquisition cohorts use first-user; per-visit mix uses session — and do not blend them in one calculation.
- Channel applied to session-start origin
- Distinct from user-scoped first-user channel
- Mixing scopes makes totals fail to reconcile
How it appears in analytics and logs
A session channel value applies the channel rules to the session's start origin. If it disagrees with the first-user channel, the user was acquired earlier through a different channel — that is expected scope behaviour.
Diagnostic use case
Use session channel grouping for per-session acquisition mix, and keep it separate from the user-scoped first-user channel when comparing acquisition to engagement.
What WebmasterID can help detect
WebmasterID can present channel mix per session distinctly from first-touch channel, so session acquisition and lifetime acquisition are not accidentally conflated.
Common mistakes
- Blending session channel with first-user channel in one metric.
- Expecting session and user channel totals to match.
- Assuming session channel updates mid-session.
Privacy and accuracy notes
Session channel grouping derives from referrer and campaign tags at session start, not from identity. WebmasterID groups channels first-party without fingerprinting.
Related pages
- Default channel grouping
Default channel grouping is a derived dimension that maps each visit's source, medium, and campaign onto a fixed set of channels — Organic Search, Paid Search, Direct, Email, Referral, and more. GA4 applies an ordered set of rules at query time, so a visit only gets a channel if it matches the rule's source/medium pattern. Misclassification almost always traces back to untagged or mistagged campaigns.
- 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.
- 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.
- Attribution analytics
Channel mix kept correct across scopes.
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.