HOW TO USE THE USER ID FEATURE IN GOOGLE ANALYTICS

Summary from the TLDR Marketing newsletter

The User ID feature in GA4 lets you track individual users across sessions and devices by linking an internal identifier from your CRM or database, but it requires a login system. To implement it correctly, assign each user a unique, consistent, non-personal ID, send it only after login, and reset it to null on logout. Avoid empty or invalid values, ensure user consent, and don't use User ID as a custom dimension to prevent data issues. When configured with the correct reporting identity, GA4 can retroactively connect pre-login events and enable more accurate tracking and segmentation.