What applies from this traffic sign onwards?

This yellow town sign indicates you are entering the built-up area (Ortsgebiet) of a town or city in Germany. From this point, specific traffic rules apply: you must not use your horn to signal your intention to overtake, and parked cars may be marked with parking lights. These rules are designed to reduce noise and increase safety in urban areas, where traffic and pedestrians are more frequent. Parking with only the parking light on is permitted in some urban settings. You answered correctly, as these are the rules that apply starting from this sign. Driving with only the parking light in darkness, however, is not allowed except in specific situations for parked cars, so that answer was not correct.