A telemeter scale is used for measuring distances, mostly in connection with chronographs.
With the telemeter scale a distance can be measured due to a visible and audible event, e.g. during a thunderstorm. The division on the dial is based on the speed of sound in air (340 meters per second). Start the hands in the moment when you see the flash, and stop it when you hear the thunder. The hand tells you how far away the storm is. It is obvious that the telemeter function plays a special role in mountaineering.