In general, user recognition in Webtrekk is based on cookies. If a user visits your website using the "Private Mode" (Firefox, Safari) or "Incognito Mode" (Chrome), his cookies are deleting automatically when he closes the browser. If a user visits your website again afterwards, he cannot be recognized as returning user with the help of cookies.
User recognition gets more precise if the user logs in on your website: If so, a Customer ID can be passed that allows to identify the user as returning visitor.
Further information can be found in the training chapter Analyses 2 – Basic Metrics.
With the help of the Cross Device Bridge, recognition of returning visitors can be improved even without login.