A client ID identifies one browser on one device. It’s usually stored in a first-party cookie and sent with every hit. GA4 calls it client_id, and exports it as user_pseudo_id. Matomo calls it the visitor ID.
Why It Matters
It’s how separate page loads get connected to the same visitor, and therefore how sessions and returning visitors are counted. When cookies are cleared or blocked, the same person becomes a new client.
In d8a.tech
The client ID is one of the identifiers used to group hits into sessions on the backend.