What is User-ID?
A unique, persistent identifier that you generate in your own backend database and pass to GA4 to perfectly track authenticated users across devices.
While the default GA4 Client ID breaks if a user switches from their phone to their laptop, the User-ID bridges that gap.
If a user logs into your SaaS dashboard on their phone, you send their internal database ID (e.g., USR_8891) to GA4. If they log in on their laptop the next day, you send that exact same ID again. GA4 recognizes the matching User-ID and stitches the actions together under a single, unified user journey, providing perfect cross-device attribution.
Privacy Rule: Never transmit personally identifiable information (PII) like an email address or plain-text name as a User-ID. It must be an alphanumeric database string or a hashed value.
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