For which purposes is a horn allowed to be used outside built-up areas?

Outside built-up areas in Germany, you are allowed to use the horn both as a warning signal and to indicate your intention to overtake another vehicle. This is different from within built-up areas, where using the horn is generally only allowed as a warning in cases of danger and not for overtaking signals. Using the horn as a calling signal, such as attracting someone’s attention for non-traffic reasons, is not permitted. You answered correctly: signals for warning and overtaking are allowed outside built-up areas. Remember this distinction to answer future questions accurately.