What are 0 dB / 12 dB / OFF of AUX? How should each of them be used?

When you will not use AUX input, you can prevent noise inflow by setting OFF.
0 dB and 12 dB indicate the audio amplification amount.
When, for example, connecting a CDJ, select 0 dB.
When, for example, connecting the HP analog output of a portable audio player, select 12 dB because the output level will be low.