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

In German traffic rules, the horn is allowed to be used in built-up areas only as a warning signal to alert other road users of immediate danger. It should not be used as an overtaking signal or simply to call someone. Using the horn for other purposes in these areas can be considered noise pollution and is prohibited by law. This rule helps maintain safety and reduce unnecessary noise in residential and urban areas.