The concept is that if you calibrate your system, zero represents "reference" level which is often 75, 80 or 85 db, depending on the receiver. PLuses or minuses represent the deviation from your preset reference level in dbs which is the scale used to measure sound pressure (loudness).

However, I think you said you already understood this but what I really wanted to say is that the whole zero = reference level is kind of ruined by the fact that music and movies don't get recorded at exactly the same playback volumes so one CD at zero could be loud while another could be very soft, requiring adjustment.

Too bad they wouldn't standardize, at least for new. Or add a calibration code to the beginning of digital recordings that equalizes the SPL to match a standard reference level. Hey, you didn't read that. I'm off to the patent office. Well, once I invent it.....

With great power comes Awesome irresponsibility.