GA4 Dimensions for Sources: Traffic Source vs Attribution and Manual UTM Fields

GA4 Dimensions for sources: Traffic Source vs Attribution

When looking at “source”, “channel”, or “campaign” in GA4, it can feel like the same field is used everywhere. In practice, different layers of data are involved. One layer answers where the session or the user came from, and another layer answers how GA4 will show key events and revenue in attribution reporting. Both can be viewed in standard Reports, and both can be combined in Explore, where dimensions and metrics are selected to build custom tables and charts.

What scopes are and why they matter

Scope is what the value of a dimension is attached to. In GA4, four scopes are encountered most often.

• User scope. The value is attached to the user and, for traffic sources, typically means which source acquired the user the first time. In the interface these are dimensions with the First user prefix.
• Session scope. The value is attached to a specific session. In the interface this is the Session prefix. In one session, an event has one session source, but in another session the same user may have a different one.
• Event scope. The value is attached to a specific event. For sources, this is typically dimensions without a prefix like Source, Medium, and Campaign in the attribution context.
• Item scope. This is the ecommerce product line item inside the purchase or cart level. It is relevant when analyzing the items array in ecommerce events (purchase, add_to_cart) and creating item-scoped parameters or dimensions such as “item color”.

Events, key events, and why Event count is about facts

GA4 is event-based, so almost everything is measured as an event. Event count is a raw factual counter. Key events are events marked as especially important for the business. “Credit” is an attribution term used in attribution reporting.

• Event count. How many times the event fired. This is a raw factual counter.
• Key event. An event marked as especially important for the business (previously called a conversion). The Key events metric counts how many times key events were triggered.
• “Credit”. An attribution term that describes how attribution reporting assigns contribution for important user actions across touchpoints along the user’s path. This explains why decimals can appear in attribution reporting.

Two groups of dimensions: Traffic Source and Attribution, and where they live

In GA4 there are two practical pools of dimensions for sources. Both can be used in standard Reports and combined in Explore.

Traffic Source dimensions

Traffic Source dimensions are used for acquisition analysis: where users and sessions came from. They are heavily used in standard Acquisition reports, and the same dimensions can be used in Explore.

Attribution dimensions

Attribution dimensions are used for analyzing key events and related metrics in the context of attribution settings. Attribution settings live in Admin and affect how key event reporting behaves in the relevant places.

Traffic Source: what User dimensions and Session dimensions mean

Traffic Source is the layer used to answer questions like which sources generated purchases, which sources bring new users, or how the traffic mix changes.

User dimensions (First user …) answer which source acquired the user the first time. These are user-scoped values, so they naturally appear in the User acquisition report.

Session dimensions (Session …) answer where a specific session started. These are session-scoped values, so their natural home is the Traffic acquisition report.

The key point is that the same metrics (for example, Key events and Total revenue) can be used in both reports, but the breakdown by source differs because outcomes are grouped either by the user’s first source or by the source of the session where the event happened.

User Path 1
Day 1: Facebook Ads → visit with no key event
Day 3: Google Organic → key event, revenue = 200

In Traffic acquisition (grouped by Session source/medium), the purchase and revenue appear under google / organic, because the event happened in the Day 3 session whose source is Google.

In User acquisition (grouped by First user source/medium), the same purchase and revenue appear under facebook / …, because the user was first acquired by Facebook. This is what the report is designed to answer: which channels acquired users who later performed important actions.

User Path 2
Day 1: Google Ads → key event, revenue = 200 (in the same session)

Here the picture matches in both acquisition reports. Both Traffic acquisition and User acquisition show the key event and revenue under Google, because the user’s first source and the converting session’s source are the same.

User Path 3
Day 1: Facebook Ads → visit
Day 2: Email (UTM) → visit
Day 3: Direct → key event, revenue = 200

In GA4 acquisition logic, Direct often does not “win” as the source if there are recent identifiable campaign sources, because GA4 tries not to lose known source information. In practice, this means that in Traffic acquisition the conversion often does not appear under “direct”, but under the last known session source (for example, email). In User acquisition, it is still attributed to the user’s first source (facebook). This reflects the difference between session scope and user scope.

Attribution: dimensions without prefixes vs Manual, and how this differs from Traffic Source

Inside the attribution world, two distinctions matter: the canonical source/medium/campaign dimensions and the manual values taken literally from UTM parameters.

Manual Source/Medium/Campaign is the UTM truth

Manual source, medium, and campaign dimensions correspond directly to UTM parameters on the URL. The practical value of manual fields is straightforward: they allow tagging to be audited and URL labeling to be seen exactly as it was sent. They are useful for strict naming conventions, affiliates and partners, email, QR codes, or any traffic where platform integrations are not relied on.

Source/Medium/Campaign without Manual is the cross-channel version

Traffic-source dimensions can be filled from multiple signals available to GA4, including platform integrations, auto-tagging identifiers, referrers, and UTMs. In practice, non-manual values reflect the strongest signals GA4 has available, while manual reflects UTM literally.

User Path 1
Day 1: Facebook Ads
Day 3: Google Organic → key event, revenue = 200

In acquisition reports, the result is either everything under google/organic (Traffic acquisition) or everything under facebook (User acquisition), as described in the Traffic Source section above.

In Advertising → Attribution (and in explorations using the relevant event-scoped attribution dimensions and metrics), GA4 can show values differently, and decimals are explicitly allowed under data-driven attribution. The documentation confirms the existence of decimals, but does not promise any specific split.

User Path 2
A Google Ads click with auto-tagging (gclid), but UTMs are also added, for example utm_source=facebook&utm_medium=paid_social.

In this case, manual fields show “facebook / paid_social” because that is literally what the UTM says. Meanwhile, the non-manual fields may reflect Google/Google Ads due to auto-tagging and integration signals. Manual fields are useful for tagging audits, and non-manual fields are often better for consistent channel classification across integrated platforms.

User Path 3
An email campaign with UTMs and no platform integration or auto-tagging identifiers.

Then manual and non-manual values typically match, because the strongest available signal is the UTM tagging. In acquisition reports and explorations, the same source labels appear, with the difference coming from scope (session vs first user).

How to use Source, Medium, Campaign

For a straightforward “events and revenue by source” view, start with Traffic Source dimensions and choose which question is being asked.

If the question is “in which sources did purchases happen”, use Traffic acquisition (Session …). If the question is “which sources acquired users who later purchased”, use User acquisition (First user …). If the question is “what is actually in UTM tagging”, use manual dimensions. If the question is “why decimals appear in key events and revenue and where they come from”, use Advertising → Attribution and Attribution settings.