User registration at Webtrekk initially works via cookies. A cookie is set in the browser so the user can be recognized at the next visit.
This ID can be viewed in the cookie itself (see e.g. Which cookies are set by Webtrekk?), is transferred in the pixel request (see e.g. Review of Data Collection - Parameter Overview) and is found in the Webtrekk tool as the dimension "End Device visitor IDs".
You can also transfer your own IDs. If a user logs on to the website, a unique ID for this user can be transferred to Webtrekk. This can be seen in the pixel request (see e.g. Review of Data Collection - Parameter Overview) and can be found in the Webtrekk tool as the "URM - Custom Visitor ID" dimension.
With the help of the own visitor ID it is possible to merge different devices of a user into a Webtrekk Visitor and thus have a user-centered instead of a device-centered reporting.
Webtrekk needs an internal ID for each visitor, regardless of whether they have a CustomerID or not. This is the visitor ID. It is not available in the browser because it is generated internally by Webtrekk.
The first time the user visits with a new device, a user cookie with the corresponding ID is generated (see above). If the Customer ID is handed over during the first visit of a device and the user is already known to Webtrekk, the device is assigned to the existing visitor ID. Otherwise, the visitor ID of the terminal device corresponds to the visitor ID.