What does the audio line level refer to?

Factual Questions
1

This was something that I was wondering just as an aside, but after 20 minutes of searching yielded only the same frustrating results, it began to bother me.

Now I’ve found many places on the web that detail exactly what line level is, and even a little about how it got to be the way it is. Briefly, what I’ve learned is (and I hope for correction if i’m wrong):

‘Professional’ line level is referred to as +4 dBu, with dBu referenced to 0.775 V[sub]rms[/sub] (or rather sqrt (.600 V[sup]2[/sup]), since it’s 1 mW into 600 ohms). +4 dBu is 1.23 V[sub]rms[/sub].

And there’s ‘consumer’ line level, which is -10 dBV ( .316 V, or -8 dBu if my calculation’s correct).

To be honest, I don’t care what those actual levels are ( I just stated them so that no one would think that’s what I’m looking for).

What I want to know is, what do these levels actually correspond to? Is this a minimum, a maximum, some intermediate value reference? There is a considerable range to the signals involved, so are there more standards than just that or is it purely a matter of choice with the level being a nominal value for design, and if your equipment doesn’t match mine, we just adjust the gain somewhere?

2

Those levels correspond to 0 VU, that is, the point of 100% modulation, or in more common terms, the maximum distortion-free signal allowable on the system.

3

Concur with Jovan. It’s a reference point. The input level (+4dBu, -10dBv) then corresponds with 0VU on your meters. The signal comes in, you ZERO your meter to 0VU.

This way, you can send that recording “around the world” if you want and the recipient with take the reference tone (input level), THEY will then ZERO their meters to 0VU and you can then be sure that the two of you are “hearing” the same thing.
Analog anyway.