You are extremely dense and totally incapable of understanding any of this. I said
IF it has channel trims then you could perform a calibration and then - just as in the case of a surround receiver - the number on the volume display would have meaning with respect to the
output SPL.
You took that and ran with that and said you were trying to explain to me that it doesn't have channel trims. Great, it's a stereo receiver and that particular one doesn't have channel trims. It's irrelevant as it has nothing to do with the numbers on the volume display and simply means that there is no way to determine the output SPL just by looking at the display as you could if you had done a calibration.
You replied 'NO' to my saying the receiver has a fixed crossover that is either 80 Hz or 90 Hz even when you said in the original post that you thought Onkyo said it was fixed at 80 Hz. Then Seth=L said the exact same thing but was kind enough to offer even further detail. Once you read Seth's reply you say wow, thanks that's what I was trying to explain to MDS.

You're dense!
This is a PERFECT example of what I am talking about. Despite numerous attempts to explain a simple concept, once again you reply saying we haven't gotten to the bottom of anything and ask the original question AGAIN - why does the relative scale exist?
For the umpteenth time: the numbers on the volume display are a
scale - a range between minimum and maximum. It has nothing to do with SPL, unless you calibrate, but of course it is a stereo receiver which has no facilities for calibration, so you conclude there MUST be something magical about it because in your mind a relative scale only applies to a surround receiver where '0' is a point on the scale
near max, but not AT the max. It damn sure doesn't have anything to do with THX and a 'reference' level can be anything you want - it is after all a level to which everything else is
relative.
Stereo receivers have used relative scales since the '70s. I asked if '0' was the top of the scale on the Onkyo. As I said earlier, if '0' is the top of the scale, then it is MAX - meaning maximum output (or zero attenuation of the level of the incoming signal). If the scale were absolute and the max number is '80' or 'Z' or the greek letter Delta,
it is still the same.
I pointed out that you can tell that the receiver is at 80% of max level if the absolute scale shows 80 when the top of the scale is 100 or when the relative scale shows -20 if the range was -100 to 0. If the scale were a - ZZZ, ZZZ would be max and it would be the same thing - just nonsensical because humans see 'volume' as a numeric quantity. Peng said the same thing and yet once again you just reiterate the original question and say 'why'.
I'm sure at some point you'll ask why '0' for a tone control like bass or treble means 'no change' and why does it use a 'relative' scale from -10 to +10 with zero in the middle. That can be explained in one sentence and yet you'll turn it into a multi-page thread, each time reiterating the original question!