What does this traffic sign indicate?

You answered correctly. This yellow sign with “U” and a number indicates a numbered diversion or detour route. In Germany, “U” stands for “Umleitung” (detour), and these signs guide drivers along a temporary alternative route, usually due to roadworks or closures. It is not for a tram line or an underground car park. Recognizing this helps you follow the correct detour when usual routes are unavailable.