Park Distance Control.

With Park Distance Control, entering or leaving the tightest parking space becomes much easier. An acoustic warning signal lets you know how close you are to the car or object behind you.

Ultrasound sensors integrated into the bumper at the rear of your BMW measure the distance to the nearest large object behind the vehicle. As you reverse closer, the frequency of the warning tone increases. When you're just 30 cm from the car or other obstacle, an unbroken warning tone sounds, as a signal to brake.
Depending on the model and equipment of your BMW, the acoustic signal is accompanied by a visual representation on the Control Display, so you can both see and hear where obstacles are each time you enter or leave a parking space.