Modern home-theater audio equipment use a logarithmic volume scale, measured in dB. The old-style linear volume controls which indicate a fraction of the equipment's maximum power are no longer common.
After you've run the Audyssey calibration, a volume setting of 80 (or 0 if you set the receiver to use relative values) produces the same reference sound level at your primary listening position as you would hear in a calibrated commercial movie theater. In a home environment, most people listen to movies at levels about 10-20dB below reference (i.e. about 60-70) , and to music about 20-30 dB below reference (i.e. 50-60). While there is a standard for the sound levels used in movies, music recordings do not have an equivalent.
Marantz SR7009 avr + MM9000 amp --> Atmos 7.1.4
Fronts=NHT 2.9+AC2, FH+TM=DefTech PM1000, LCR+TM amped