Every visit to your website starts somewhere: a search result, a social post, an email, an ad, a link on another site, an AI assistant's answer or someone typing your address. Those starting points are your traffic sources, and knowing which ones bring valuable visitors is the foundation of every budget and content decision.
Google Analytics 4 organises this information in three layers, source, medium and channel, and then groups channels using rules most people never read. In 2026 those rules gained a new AI Assistant channel and a Source Group field, so even experienced marketers find a few surprises.
This guide explains the vocabulary, walks through every default channel, shows where to find each one, explains how to build a custom channel group and gives a framework for judging whether your traffic mix is healthy. For deep dives on specific channels, see our guides to organic traffic, direct traffic and referral traffic in GA4.
Key Takeaways
- Source is where a visit came from (google, newsletter); medium is how (organic, email); a channel is a rule-based group of both.
- GA4's default channel group has around 20 channels, including AI Assistant added in May 2026.
- Channel definitions in the default group can't be edited; build up to 2 custom channel groups on standard properties instead.
- Custom channel groups apply retroactively, and the first matching channel wins.
- Judge sources by engaged sessions, key events and revenue, not by sessions alone.
Source, medium, campaign and channel
| Field | Meaning | Example values | Set by |
|---|---|---|---|
| Source | The origin of the visit | google, bing, facebook.com, newsletter | Referrer or utm_source |
| Medium | The type of traffic | organic, cpc, referral, email, (none) | GA4 rules or utm_medium |
| Campaign | The specific promotion | spring_sale, (organic), (referral) | utm_campaign or ads auto-tagging |
| Default channel group | Google's rule-based grouping | Organic Search, Paid Social, Email | GA4, based on source/medium/campaign |
| Source platform / Source group | The ad platform or consolidated source | Google Ads, Manual; Facebook (all variants) | GA4 |
The Source Group field, added in June 2026, consolidates variants like m.facebook.com and l.facebook.com into one value, which makes social reporting less fiddly.
Three scopes, three answers
The same visit can show up differently depending on the dimension's scope. Google explains that First user dimensions describe how a user was first acquired, Session dimensions describe each visit, and event-scoped dimensions (no prefix) credit key events using your attribution model.
“Event-scoped dimensions don't include a prefix, as in Source or Medium.”
— Google Analytics Help, Scopes of traffic-source dimensions
Use User acquisition to see what introduces people, Traffic acquisition to see what brings visits, and Advertising attribution reports to see what earns conversions. Our marketing attribution guide explains the models.
Every default channel, explained
| Channel | What lands here | Typical example |
|---|---|---|
| Organic Search | Unpaid clicks from search engines | google / organic |
| Paid Search | Search ads | google / cpc |
| Direct | No known source | (direct) / (none) |
| Referral | Links from other websites | partner.com / referral |
| AI Assistant | Recognised AI assistants | chatgpt.com / ai-assistant |
| Organic Social / Paid Social | Social networks, unpaid or paid | instagram.com / referral; facebook / paid |
| Tagged email links | newsletter / email | |
| Organic Video / Paid Video | Video sites | youtube.com / referral |
| Organic Shopping / Paid Shopping | Shopping sites and free listings | Google Shopping free listings |
| Display | Banner and display ads | medium display, banner or cpm |
| Cross-network | Campaigns across networks | Performance Max, Demand Gen |
| Affiliates, Audio, SMS, Mobile Push Notifications | Specific mediums | affiliate, audio, sms, push |
| Paid Other / Unassigned | Paid mediums that fit nowhere else / no rule matched | utm_medium=partner |
Google is explicit that these default definitions are fixed and aren't case sensitive. A large Unassigned bucket nearly always means non-standard UTM values; fix them with a consistent naming convention from our UTM parameters guide.
How to see your traffic sources in GA4
- Go to Reports > Acquisition > Traffic acquisition.
- Set a date range and compare to the previous period or year.
- Keep Session default channel group as the primary dimension for the overview.
- Switch to Session source / medium to see individual sources, or Session campaign for campaigns.
- Choose a key event in the Key events column to compare conversion performance.
- Repeat in User acquisition to see which channels bring new users.
Building a custom channel group
When the default group doesn't fit, for example to split AI traffic your way, separate branded paid search or give partners their own channel, create a custom group.
“Custom channel groups can be applied to your reports retroactively.”
— Google Analytics Help, Custom channel groups
- Go to Admin > Data display > Channel groups (Editor role or above).
- Click Create new channel group; it starts as a copy of the default group.
- Name it, then add or edit channels with conditions on source, medium, campaign or source platform.
- Reorder channels: traffic is placed in the first channel it matches, so put specific channels above general ones.
- Save, then select the custom group as the dimension in Traffic acquisition or explorations.
Standard properties get up to 2 custom groups (360 properties up to 5), each with up to 50 channels. Our guide to tracking AI traffic in GA4 includes a worked custom-group example.
Is your traffic mix healthy?
There's no universal “right” mix; a local plumber and a SaaS start-up will look completely different. Instead, ask these questions:
- Concentration risk: does one channel deliver most key events? Algorithm or policy changes could hit you hard.
- Owned vs rented: how much comes from channels you control (email list, direct, organic) versus paid platforms?
- Quality: which channels have the best engaged-session and key event rates?
- Trend: which sources are growing year over year, and which are flat despite investment?
- Cost: what does each channel cost per lead or sale? See our guide to cost per acquisition.
Combine the answers into a simple scorecard in a marketing dashboard and review it monthly.
Common mistakes with traffic sources
- Inconsistent UTMs. “Facebook”, “facebook” and “fb” split one source three ways and push some into Unassigned.
- UTMs on internal links. They overwrite the real source mid-visit.
- Comparing User and Traffic acquisition totals. Different scopes, different numbers.
- Ignoring Unassigned. It's a to-do list for tagging fixes.
- Treating direct as brand. Much of it is untagged marketing.
- Optimising for sessions. A channel with fewer, better visits often wins on revenue.
Related Guides
- Cohort Analysis: How to Measure Retention and Customer Value
- Data-Driven Marketing: A Practical Framework for Small and Mid-Sized Teams
- How to Build a Marketing Dashboard
Frequently Asked Questions
What are the main website traffic sources?
The main channels are organic search, paid search, direct, referral, social (organic and paid), email, display, video and, since 2026 in GA4, AI assistants.
What is the difference between source and medium?
Source is where a visit came from, such as google or a newsletter name. Medium is the type of traffic, such as organic, cpc, referral or email.
Why is some traffic Unassigned in GA4?
No default channel rule matched its source, medium or campaign, usually because of non-standard UTM values.
Can I edit GA4's default channel group?
No. You can create custom channel groups instead: up to 2 on standard properties and 5 on Analytics 360.
Which traffic source is best?
The one that delivers the most valuable key events at an acceptable cost for your business. Compare channels on conversions and revenue, not sessions.
Conclusion
Traffic sources tell you where attention and revenue come from. Learn the source/medium/channel model, keep your UTMs disciplined, add a custom channel group when the defaults don't fit and judge every channel by key events, not visits. If you'd like a clear view of which channels pay and a plan to grow them, explore our digital marketing services or request a free quote.
References
- [GA4] Default channel group – Analytics Help
- Custom channel groups – Analytics Help
- Scopes of traffic-source dimensions – Analytics Help
- [GA4] Traffic acquisition report – Analytics Help
- What's new in Google Analytics – Analytics Help
- Google Analytics Traffic Sources: An In-Depth Guide – Semrush Blog
