What do the -20dB, -10dB, and -5dB LEDs on the front panel indicate?

The -20dB LED lights when the output signal is around -20 dB below full power, the -10dB LED lights at around -10 dB below full power, and the -5dB LED lights when the output signal is around -5 dB below full power. These serve as output level meters for each channel. (Page 2)