The reviewer should mention in his/her review the approximate average and peak volume he/she is listening to when giving the subjective opinion.
If he/she does NOT mention this, then he/she just forgot to mention it.

We would assume it is the reviewer's subjective comfort listening level. Very subjective opinion.
Now in terms of calibration, home calibration is usually set to 75dB because most people find pink noise above 75dB to be very loud and annoying. Commercial theaters usually calibrate to 85dB level. But that is just for calibration.
For measurements, you can see the graphs on the Y-axis in terms of volume level in decibel (dB).
But, again, for the actual subjective listening impressions, the reviewer needs to mention the volume he/she is listening to.