Attribution
UTM medium
Also called utm_medium.
UTM medium is the parameter that names the channel family, such as email, cpc, or social. It is written utm_medium.
How it is measured
The value is read from the address and combined with the source. Channel grouping rules usually key off the medium, so standard values matter more than the other tags.
Use a short fixed list, such as email, cpc, social, display, and referral. Anything outside the list will probably fall into an unmatched channel.
Worked example
A cycling-apparel brand finds that its ad agency tags paid social clicks with medium 'paid-social', the in-house team uses 'social', and the influencer program uses 'influencer'.
Reports show three rows that behave differently in the channel grouping: Paid Social, Social, and an unassigned bucket with 740 sessions. The brand agrees on 'paid-social' and 'influencer' as rules and adds both to the grouping.
How it differs
A UTM medium names the channel kind. A UTM source names the specific platform or sender. Medium is cpc; source is google.
Common errors
Using the platform name as the medium. Mixing case. Inventing new values per campaign. Using medium for the offer. Leaving the field out so the session falls into direct.
In practice
Write the allowed medium list and share it with agencies. Check the channel grouping rules for each value. Reject links with new values until the rules exist.