If the subject has been covered already, apologies.
What is trained user exactly? What kind of training makes you a trained listener? How do you acquire it?
I am genuinely asking because I see this figure referenced quite often but I don't know what it means, exactly.
Thank you.
How do you get to Carnegie Hall? Practice, practice, practice.
The more time you spend listening intently, both to test tones and familiar music recordings, and the more you try to correlate what you hear with objective measurements, the better you can learn to discern both non-linear distortions and departures from linear response. My former dealer/repair tech (Jim Ott of NW Audio Labs, now retired and defunct respectively) could listen to a speaker briefly and say "there's about a 3dB dip from around 800 to 2,200 Hz" or "the treble is goosed up about 1.5dB above 3kHz" or whatever, and measurements invariably (with greater precision) confirmed his observations. Likewise he was hyper-sensitive to noise and distortions, and could hear an amp just beginning to clip at 0.5% THD, or the slightest ground loop hum. All because he had spent so many years at it. I'm not at his level, but in 35 years of DIYing speakers, I'm getting a lot closer. I know within a few seconds if something doesn't sound quite right, and have a pretty good idea of the frequency range(s) of the problem before I run measurements.
Once you dial in your system for a fairly flat response at listening position, you can play with EQ and learn what bumps and dips sound like at different frequencies. This is easiest with pink noise, but eventually carries over to music.
The other thing is the quality of your natural equipment. Some people hear better than others from birth, just like some see better. Only a small percentage of the population, mostly musicians, have perfect pitch. But even they can benefit from constant practice, and others can learn.