What should a team validate first when channel group definitions in ga4: the complete default channel reference appears?

Reproduce the problem in the live implementation, isolate whether it is scoped to one report or flow, and compare it against at least one secondary source before changing the setup.

How do I know whether the fix actually worked?

You need before-and-after evidence in the browser and in the downstream report. A clean-looking dashboard without validation is not enough.

When should this become a full GA4 audit instead of a quick fix?

If the issue touches attribution, consent, revenue, campaign quality, or data trust for more than one workflow, it is usually safer to audit the surrounding implementation than patch only the visible symptom.

Channel Group Definitions in GA4: The Complete Default Channel Reference (2026)

How many default channel groups does GA4 have?

GA4 has 18 default channel groups in 2026: Direct, Organic Search, Paid Search, Organic Social, Paid Social, Organic Video, Paid Video, Display, Email, Affiliates, Referral, Audio, SMS, Mobile Push Notifications, Organic Shopping, Paid Shopping, Cross-network, plus Unassigned as catch-all. GA4 evaluates every session top-down, first match wins. Most attribution issues come from sessions landing in Unassigned (no rule matched) or AI engines defaulting to Referral because GA4 has no AI Assistants channel.

The 18 channels and their rules

Each channel matches based on source, medium, campaign name, and sometimes click ID. GA4 evaluates rules in a fixed order — first match wins.

#	Channel	Match logic
1	Direct	Source = (direct) AND medium = (none) or (not set)
2	Cross-network	Campaign name contains cross-network (Performance Max, Smart)
3	Paid Shopping	Source matches shopping list AND medium matches paid pattern
4	Paid Search	Source matches search list AND medium matches paid pattern
5	Paid Social	Source matches social list AND medium matches paid pattern
6	Paid Video	Source matches video list AND medium matches paid pattern
7	Display	Medium = display, banner, expandable, interstitial, cpm
8	Paid Other	Medium matches paid pattern (no source match above)
9	Organic Shopping	Source matches shopping list OR campaign matches shopping pattern
10	Organic Social	Source matches social list OR medium = social variants
11	Organic Video	Source matches video list OR medium contains video
12	Organic Search	Source matches search list OR medium = organic
13	Email	Source or medium = email, e-mail, e_mail, e mail
14	Affiliates	Medium = affiliate
15	Referral	Medium = referral, app, link
16	Audio	Medium = audio
17	SMS	Source = sms OR medium = sms
18	Mobile Push	Medium ends with push, contains mobile/notification, OR source = firebase
19	Unassigned	No other rule matched

The site lists Google maintains

Three rules above depend on Google's internal site lists:

Search list — google, bing, yahoo, baidu, yandex, duckduckgo, ecosia, naver, seznam, ask, kagi, you.com, startpage, brave, qwant. Notable absences: AI assistants (chatgpt.com, perplexity.ai, claude.ai). They do NOT match Organic Search by default — they fall through to Referral.

Social list — facebook, instagram, twitter/x.com, linkedin, tiktok, pinterest, reddit, snapchat, whatsapp, threads, mastodon, bluesky, discord, slack. Reddit was added relatively recently; older properties may still see it as Referral.

Video list — youtube, vimeo, twitch, dailymotion, ig.me. TikTok matches both Social and Video lists.

Shopping list — amazon, walmart, target, ebay, etsy, shopify storefronts, alibaba, mercadolibre.

These lists aren't fully publicly documented and can change without notice. Any source not on Google's lists falls through to Referral or Unassigned — which is why custom channel groups exist.

Why sessions land in Unassigned

Five most common causes:

1. Custom mediums that don't match any rule. A campaign tagged with medium=newsletter doesn't match Email (which requires exact values email, e-mail, e_mail, e mail). Falls through to Unassigned.

2. Casing inconsistencies. Medium=Email vs email — GA4's matching is case-sensitive in some contexts.

Want to see whether attribution loss is already distorting your channel data?

Audit attribution

3. Source present but medium missing. A referral domain with no medium (source=stripe.com, medium=(not set)) doesn't match Referral. Lands in Unassigned.

4. New AI engines without proper handling. ChatGPT, Perplexity, Claude, Gemini show up in Referral by default — but if their domain isn't recognised, they can land in Unassigned.

5. Custom-built mobile app sources. Custom-source mobile traffic can land in Unassigned if medium doesn't match any rule.

The fix: look at your top 20 Source/Medium combinations in Unassigned. Most are fixable by either correcting UTM convention at source, or by building a custom channel group that catches them with explicit rules.

The custom channel group escape hatch

Default channels can't be edited. To handle non-standard traffic, build a custom channel group:

GA4 Admin → Data display → Channel groups
Copy the default group (you can't edit the default)
Add custom channels — for example: AI Assistants (source matches AI regex), Podcast (source contains podcast OR campaign contains pod-), Internal Test (source = internal-test for QA)
Order matters. Place custom channels above the rule they'd otherwise be caught by. AI Assistants must sit above Referral.
Save. Standard tier gets one custom channel group; 360 gets two.

Custom channel groups apply retroactively to standard reports — historical sessions get reclassified the moment you save. They do NOT apply retroactively to BigQuery exports.

How to debug a session in the wrong channel

Six-step diagnosis:

Confirm Source/Medium values in GA4 → Reports → Acquisition → Traffic Acquisition.
Look up the channel matching rule in the table above.
Check the rule order. Higher-priority rules can intercept before the rule you expected.
Check casing or punctuation. CPC vs cpc can produce different results.
Check the site lists. If Source isn't on Google's recognised list for that channel, it won't match.
Check for click IDs. gclid forces Paid Search regardless of explicit medium.

The most subtle bug: a campaign tagged correctly with medium=email but landing in Referral because the Source matches a domain Google's referrer-detection has classified as a referrer. Fix at source: strip the referrer using GTM's URL Passthrough or set medium=email explicitly via UTMs.

Channel ordering as strategy

Paid AI ahead of AI Assistants. Two custom channels: Paid AI (medium=paid_ai) and AI Assistants (source matches AI regex). Place Paid AI first so paid clicks don't get bucketed into the organic AI channel.

Branded Search separate from Organic Search. A custom channel for Branded Search (campaign contains your brand name) above Organic Search lets you separate brand from non-brand without rebuilding attribution.

Internal traffic above everything. A custom channel for internal traffic (medium=internal or hostname matches your testing domains) above all other channels prevents internal QA from polluting real channel reporting.

The principle: build custom channels that match before the default rules they'd otherwise be caught by. Order is your only lever for changing default channel behaviour.