Source and medium
In plain English: Source is where traffic is associated; medium is how it arrived.
google / organic commonly represents an unpaid Google Search visit. newsletter / email can identify a tagged email campaign. example.com / referral can describe a link from another site.
- google / organic
- newsletter / email
- example.com / referral
Channel groups
In plain English: Channels are rule-based labels built from source and campaign information.
Common values include Organic Search, Direct, Referral, Organic Social, Paid Search and Email. Google's default channel definitions can change, so use current documentation when diagnosing classification.
Direct is a fallback
In plain English: Direct means GA4 has no usable referring or campaign information for that traffic.
Typing a URL or using a bookmark can produce Direct traffic, but missing tags, privacy controls, apps and untagged documents can also remove attribution context. Do not interpret Direct as one behaviour.
User vs traffic acquisition
In plain English: User acquisition describes how a user was first acquired; traffic acquisition describes sessions.
First user dimensions are user-scoped. Session dimensions describe the source associated with a session. Their totals and attribution questions differ, so compare like with like.
Keep in mind: Attribution is a reporting model, not a complete record of every influence on a decision.