analytics.identify() when you want future extension events to include an account identifier:
Track and identify
Identify users
Associate future events with an account identifier.
Use
Choose a stable, opaque account ID where possible. You are responsible for the values you send and for describing your data collection in your privacy policy. The method accepts a nonblank string of up to 128 Unicode characters without control characters.
When a user signs out, clear the identity for future events:
Identifying or clearing an account does not create an event. Earlier events keep the identity they had when recorded; anonymous events are not assigned retroactively. Analytics counts each extension installation separately, even when the same account is used on multiple installations. Clearing an identity does not delete past data.