If you switch it on it uses the residual heat to keep the car warm when you stop. I think it is intended to keep the car warm for a short stop.
The middle "REST" button is the the residual heat control. This button is the "Immediate pre-entry climate control". The icons are totally mystifying, but I found this in the manual:
![IMG_0125.jpg IMG_0125.jpg](https://forums.mbclub.co.uk/data/attachments/35/35424-f2f316b2fae0bc8e7282212565f412a3.jpg?hash=8vMWsvrgvI)