500mV (0dB) on the voltmeter?

I am tuning up my Nakamichi CR-3A cassette deck and am trying to understand the meaning of a dB value that follows a mV value. In relation to measuring output with a 400Hz input, the service manual refers to obtaining "500mV -2dB on the AC voltmeter" from the output jacks. In another spot it refers to obtaining "500mV (0dB) on the AC voltmeter" with a 19kHz input. I am using a Brymen 867 multimeter. I can measure 500mV from the output jacks easily enough but am left with two questions: 1) what do these dB values refer to? 2) is there any difference between the dB value within or without parentheses?