How many stops of light an image occupies, and how much dynamic headroom is left. Enter the system’s bit depth and the raw shadow and highlight values, and the tool works out the stops occupied by the scene and those still available, split between shadow and highlight headroom, on a logarithmic ADC scale. It’s the bridge between the way the eye thinks about light — in doublings, that is, stops — and the way the sensor records it — in the linear levels of an analog-to-digital converter.