A user ID is your own identifier for a logged-in user, set by you in the tracking call: user_id in GA4, uid in Matomo. Trackers don’t create it automatically.
Why It Matters
It lets you connect activity across devices and browsers once someone signs in. It is personal data, so set it deliberately and document the legal basis.
In d8a.tech
By default, a change of user ID splits a session, so two people sharing a browser don’t merge. Joining sessions across devices by user ID is optional.