UTM parameters look like a five-minute job. Add three values to a link and GA4 knows where the visit came from. The trouble starts a few months later, when one campaign appears under Facebook, facebook and fb, and a chunk of paid traffic sits in the wrong channel. The tags themselves are simple. What holds up over time is the convention behind them.
What the parameters do
Google's guide to custom URLs says you should always use three:
utm_source: where the traffic comes from, such as a newsletter name or platform.utm_medium: the type of marketing, such asemailorcpc.utm_campaign: the promotion or product.
Optional values add detail. utm_content separates creatives or links, utm_term holds a paid keyword, and utm_id can link the traffic to uploaded campaign data. Google also says that missing parameters lead to (not set) values in reports.
Two facts shape everything else. Values are case sensitive, so Meta and meta are different sources. And GA4 builds its default channels from these values, so a small naming choice decides where the traffic is reported.
