On a multi channel receiver or pre processor, what are the volume numbers referenced to? Is there a standard?

For example, if my volume display shows -30 dB, what is it telling me for each of my channels?

The Rat. M80s, VP-150, QS8s, SVS PC 20-39+, OPPO, Onkyo 703s, Harmony 880 Sony 60" SXRD HDTV