Privacy
Client ID
Also called cid, anonymous id.
Client ID is the anonymous key a collector assigns to a browser profile so repeat visits can be linked. It is stored on the device, typically in a cookie.
How it is measured
The value is a random string created on the first hit and sent with every later hit. Google Analytics, for example, keeps one in the _ga cookie. The tool counts distinct client IDs in a period to estimate visitors.
Look for it in the request payload or in the browser's storage panel. If the same string appears after you close and reopen the browser, the ID is persistent. If it changes on every load, it is not stitching anything together.
Worked example
A hobbyist forum owner opens the site on a laptop and a phone. The analytics tool shows two client IDs, one for each browser, though one person is reading. After clearing cookies on the laptop and visiting again, a third ID appears.
In the weekly report, 5,400 client IDs get reported as visitors. The owner estimates the real number of people is nearer 4,000, since some readers use two devices and some wipe cookies.
How it differs
A client ID names a browser profile with no login attached. A user ID names a person after the site authenticates them. The client ID excludes anything about who the person is; the user ID excludes visitors who never sign in.
Common errors
Treating one ID as one human. Forgetting that it resets when storage is cleared. Calling it non-personal data when it can be linked to a profile. Logging the ID alongside an email in the same system. Assuming Safari keeps it for the stated lifetime.
In practice
Check what your analytics sets and for how long. If you keep a client ID, list it in your privacy notice and ask for consent where the law needs it. If your tool does not store one, say so plainly and do not quote visitors as people.