KOMBO 42 Receiver: Why does the volume display reach the "minus range"?

The display of the volume on a scale down to the minus range is a quite common type of display, especially for high-quality amplifiers and corresponds to the unit dBFS (Decibels relative to full scale) The value 0 on the scale corresponds to the value of full scale, i.e. the highest possible level value.

Reading tip > This is dBFS (Wikipedia)