Parameter

A parameter is a piece of detail attached to an event that describes it. If the event is the verb — something happened — parameters are the adjectives and nouns that say what, where, and how much. A page_view event carries a page_location parameter; a purchase carries value and currency. Without parameters, every event would just be a count with no context.

Why it matters

Parameters are where the useful detail lives. They turn "a purchase happened" into "a purchase worth £140 of the blue jacket happened on the checkout page." When you want to break a report down — by page, by product, by value — you're almost always slicing on a parameter. The richer your parameters, the more questions your data can answer.

A concrete example

Suppose you add a custom event for newsletter signups and attach a parameter called signup_location with values like "footer" or "popup." Now you can see not just how many people subscribed, but where on the page they did it. To use that parameter in standard reports, you first register it as a custom dimension — otherwise GA4 collects the data but won't display it as a breakdown.

The common misreading

The classic pitfall is expecting a parameter to appear in reports automatically. It won't. GA4 collects unregistered parameters but won't show them as a dimension until you register them, and only from that point forward. People send a custom parameter for weeks, then go looking for it and find nothing — because they never registered it. Set up the custom dimension before you need the data, not after.