Question

What identifiers and events are required for deterministic identity resolution in Segment Unify, and how does Segment determine when to unify or identify a user profile? 

Product

Twilio Segment

Environment

Segment Console

Answer

To achieve deterministic profile unification in Segment Unify, at least one stable identifier—such as userId, email, or another unique, consistently assigned value is required. These identifiers allow Segment to reliably link user activity across sessions and platforms.

Anonymous users are tracked using an anonymousId before they provide any identifying information. When a user logs in, registers, or otherwise provides a stable identifier, an identify event should be sent with that identifier. At this point, Segment can merge the anonymous activity with the identified profile, unifying the user’s history.

For two sessions or touchpoints to be stitched into a single unified profile, they must share at least one stable identifier. If only anonymousId is present and no stable identifier is ever sent, those sessions will remain separate and cannot be deterministically unified.

Send an identify call with a stable identifier as soon as possible in the user journey and use consistent identifiers across all platforms (web, mobile, server) to ensure reliable unification.