Google Analytics 4 (GA4) organizes your incoming traffic into 18 default channel groups, each representing a distinct source category like Organic Search, Direct, Paid Social, or Referral. Beyond these built-in categories, you can define your own custom sources using UTM parameters on your URLs or by linking ad platforms like Google Ads directly to your property.
How GA4 Categorizes Traffic Sources
GA4 uses three core dimensions to describe where a visitor came from: source, medium, and channel group. The “source” is the specific origin of the traffic, such as “google” or “facebook.” The “medium” describes the general type of traffic, like “organic” or “cpc” (cost per click). The “channel group” is a broader bucket that GA4 assigns automatically based on rules applied to the source and medium together.
You’ll see these dimensions in the Traffic Acquisition report, which breaks down how each session arrived at your site. Understanding the relationship between these three layers helps you read your reports correctly: a single channel group like “Paid Search” can contain traffic from multiple sources (Google, Bing, Yahoo), all sharing the same medium type.
The 18 Default Channel Groups
GA4 automatically sorts every session into one of its default channel groups. You cannot edit these built-in groups, but you can create custom channel groups with your own rules. Here’s what each default channel represents:
- Direct: Visitors who typed your URL into their browser or used a bookmark. Traffic with no identifiable source also falls here.
- Organic Search: Visitors from unpaid search engine results, including Google’s AI Overviews and AI Mode.
- Paid Search: Visitors who clicked a search ad on Google, Bing, Baidu, or other search engines.
- Organic Social: Visitors from unpaid links on social platforms like Facebook, Instagram, or Twitter/X.
- Paid Social: Visitors from paid ads on social platforms.
- Organic Video: Visitors from unpaid links on video sites like YouTube, TikTok, or Vimeo.
- Paid Video: Visitors from paid ads on video platforms.
- Organic Shopping: Visitors from unpaid product listings on shopping sites like Amazon or eBay.
- Paid Shopping: Visitors from paid product ads on shopping sites or individual retailer sites.
- Display: Visitors from display ads, including the Google Display Network.
- Email: Visitors who clicked a link in an email.
- Affiliates: Visitors arriving through affiliate partner links.
- Referral: Visitors from non-ad links on other websites, such as blogs or news sites.
- SMS: Visitors who clicked a link in a text message.
- Audio: Visitors from ads on audio platforms like podcast apps.
- Mobile Push Notifications: Visitors who tapped a push notification while not actively using your app.
- Cross-network: Visitors from ad campaigns that span multiple networks simultaneously, such as Google’s Performance Max campaigns that run across Search, Display, and YouTube together.
- Paid Other: A catch-all for paid traffic that doesn’t match the Search, Social, Shopping, or Video categories.
Custom Sources With UTM Parameters
When you run campaigns outside of Google’s ad ecosystem, GA4 won’t always categorize traffic correctly on its own. UTM parameters solve this. They’re tags you add to the end of a URL so GA4 knows exactly where the click came from, what type of marketing drove it, and which campaign it belongs to.
Three parameters are considered essential every time you tag a URL:
- utm_source: The specific platform or publisher, such as “facebook,” “newsletter,” or “partner_blog.”
- utm_medium: The marketing channel type, such as “email,” “cpc,” or “paid_social.”
- utm_campaign: The name of the specific campaign or promotion, such as “spring_sale” or “product_launch.”
A tagged URL looks like this: https://www.example.com/?utm_source=email_campaign&utm_medium=email&utm_campaign=summer-sale
GA4 also supports several optional parameters. You can use utm_content to distinguish between different links in the same message (like two buttons in one email), utm_term to track paid keywords, utm_creative_format to label the ad type (display, video, native), and utm_marketing_tactic to note targeting criteria like remarketing or prospecting. There’s also utm_id for tying sessions to a specific campaign ID and utm_source_platform for identifying the buying platform that managed the campaign.
One important detail: parameter values are case sensitive. “Facebook” and “facebook” will show up as two separate sources in your reports. Google recommends using lowercase consistently across all your tags. Skipping parameters you intended to set will cause “(not set)” values to appear in your reports, which makes analysis harder.
Sources From Platform Integrations
If you run Google Ads campaigns, you don’t need UTM parameters at all. Linking your Google Ads account to GA4 enables auto-tagging, which automatically passes detailed campaign, ad group, and keyword data into your reports. The same applies to Search Ads 360 and Display & Video 360.
When these integrations are active, GA4 populates a “Session source platform” dimension that identifies which advertising platform managed the traffic. Google Ads traffic shows as “Google Ads” in that dimension, while Search Ads 360 traffic appears as “SA360.” This gives you a cleaner view of performance across platforms without any manual tagging.
You can also link Google Search Console to your GA4 property. This doesn’t change how traffic sources appear in acquisition reports, but it adds a dedicated report showing which search queries brought users to your site, along with impressions, click-through rates, and average ranking position. It’s the only way to see the actual keywords driving your organic search traffic.
Where to Find Source Data in GA4
The primary place to explore traffic sources is the Traffic Acquisition report, found under Reports > Acquisition > Traffic Acquisition. This report defaults to showing sessions grouped by default channel group, but you can switch the primary dimension to “Session source,” “Session medium,” or “Session source/medium” for more granular detail.
GA4 also offers a User Acquisition report in the same section. The difference is subtle but important: Traffic Acquisition attributes each session to the source that started it, while User Acquisition attributes all of a user’s activity to the source that first brought them to your site. If someone first found you through organic search and later returned by clicking an email link, Traffic Acquisition would credit the email session to email, while User Acquisition would credit that user’s activity to organic search.
For deeper analysis, GA4’s Explore section lets you build custom reports combining source dimensions with any metric you want, such as conversions, revenue, or engagement rate. You can also create custom channel groups here if the 18 defaults don’t match how your organization thinks about its marketing channels. Custom channel groups let you write your own rules for how source and medium combinations get categorized, without affecting the default groups.