When it is granted, GA4 measures normally; when denied, tags send cookieless pings and Google models the gap. It is one of four Consent Mode v2 signals, alongsidead_storage,ad_user_data, andad_personalization, that your consent banner has to set correctly.

In practice

Make sure your consent banner maps its analytics category to analytics_storage accurately. If the signal is wrong, you either measure users who declined or lose measurement from users who agreed, and both are hard to spot after the fact.