Attribution
UTM source
Also called utm_source.
UTM source is the parameter that names the platform or publisher that sent the click, such as newsletter, google, or partner-name. It is written utm_source.
How it is measured
The value is read from the address and combined with the medium to form the source / medium pair. It identifies who sent the traffic, not what kind of channel it was.
Keep a list of sources and use them lowercase. Use one name per sender. If a site has two names, such as twitter and x, pick one and use it everywhere.
Worked example
A homebrew-supply shop tags the same email list as 'newsletter', 'weekly', and 'mailchimp' across three templates. The source report shows 1,100, 840, and 260 sessions in three rows.
The shop merges them to 'newsletter'. It sees 2,200 sessions at a 3.1 percent order rate and compares the list with a partner site that sends 600 sessions at 5.4 percent. The partner is the better source per visit.
How it differs
A UTM source names the sender. A referrer is what the browser reports. The tag overrides the referrer when present, so a wrong tag hides the true origin.
Common errors
Using several names for one sender. Mixing capitalization. Putting a campaign name in the source. Forgetting to tag partners. Naming the source after the tool used to send.
In practice
Make one list of source names and require it for every link. Review it monthly for duplicates. Merge old rows in reports when you clean up.