User ID

A user ID is a stable identifier the product assigns to a signed-in account so analytics can treat that person's behavior as one across devices.

The product sends this identifier with events at sign-up or login, and the analytics tool stitches it to the browser's previous anonymous identifier so pre-login and post-login behavior form one chain. The value should be an internal opaque identifier, not an email address or a phone number, because those are personally identifiable information and create legal obligations once they enter analytics. Two implementation failures are common: an identifier that changes every session, which splits one person into several, and a shared account, which merges several people into one profile. Whether stitching also rewrites past events or only applies from the moment of login differs between tools and should be settled before implementation.