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GA4 collects any parameter you send, but reports only show the ones you register. Registering a parameter as a custom dimension (under Admin → Custom definitions) makes it available in reports and Explorations.
Custom dimensions have a scope: event, user or item. Most are event-scoped. Standard GA4 properties limit how many you can create, so name them clearly and don't register parameters you'll never report on.
Data only appears from the moment you register the dimension. It doesn't fill in history.
Example
Your forms send a form_name parameter. You create an event-scoped custom dimension called "Form name" using that parameter. Within a day or two, you can break down generate_lead by form in reports.
Official documentation
- About custom dimensions and metrics (opens in a new tab) Analytics Help
- Create event-scoped custom dimensions (opens in a new tab) Analytics Help
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