The simple reality is that vendors who sell products are mostly free to post whatever specifications they like, in whatever form they like, as long as there is no
legally-provable "intent to mislead or defraud". Note that we have many laws concerning "truth in advertising" and "accuracy of product claims" - however, in most cases, they are based on product safety and liability. (For example, medicines and foods are most tightly regulated, because they have the greatest chance of causing harm if they are misrepresented.)
About the only specific legal regulations regarding how audio equipment is described occur in the original "FTC Requirements for Power Amplifier Specifications" document published at or about 1972 by the US Federal Trade Commission (who regulates interstate commerce of retail products). Recognizing that misrepresentation was very common in amplifier power measurements, and partly in response to lobbying from several "reputable audio companies", the Federal Trade Commission published a legal requirement for how the power rating on home audio amplifiers MUST be both measured and described in advertisements. This is the "standard" that is often referenced - and which refers to things like specifying power as "watts RMS continuous", including THD in the measurement, and providing a specific "pre-conditioning warmup period". Note that this paper is very limited, applies almost entirely to only "stereo audio equipment" (it's applicability to "multi-channel gear", "car audio equipment", and "pro audio gear" is extremely limited), and has been revised several times.
The reality is that, in most cases, the manufacturer is free to provide whatever specifications they prefer, in whatever form they prefer, as long as they don't "intentionally outright lie".
It's also worth noting that there are very few specifications that are actually "universally agreed upon" for audio gear.
For example, you may consider 10% THD to be totally unacceptable for an audiophile quality home audio amplifier, but it may be quiet acceptable for a stadium public address system, or for a low cost table radio or portable music player. Therefore, it's not unreasonable to suggest that it's up to the consumer to decide what specifications THEY consider to be important, and to simply refuse to purchase products whose manufacturers fail to provide those specs.
You also need to realize that published specifications are intended to sell products... usually to specific markets...
So, for example, let's say I had an amplifier that produced 72 watts at 0.01% THD, 92 watts at 0.1% THD, and 102 watts at 10% THD.
(These would be typical measurements for a medium-sized solid state amplifier.)
If I was attempting to market that amplifier to audiophiles, who strongly favor very low THD numbers...
I would probably rate it conservatively at "65 watts at 0.01% THD".
However, if I was marketing it to typical consumers, and one of my main competitors had a similar model rated at "75 watts @ 0.1%"...
I would rate mine as "90 watts at 0.1%" to emphasize the fact that it was more powerful than my competitor's product at the same rated distortion.
And, if I was selling it as an amplifier to be used with public address equipment in small theaters...
I might rate it as "a 100 watt public address amplifier" - and only mention that rating was at 10% in the proverbial fine print.
(Note that, for PA equipment, 10% THD is considered perfectly acceptable, but maximum available volume is important, and "100" is a nice round number.)
Note that NONE of those rating is incorrect, misleading, or "bad" - they just provide the information in the form preferred by the "target market".
I was surprised when noticed that this area of audio industry so much relaxed regarding official specs! Even the output power isn't specified sometimes, or specified at THD 10% that's really cheap cheat )) The same thing regarding the THD+N, I never see any conditions for that, only the number. In audio-amps area I always have to specify the THD+N and the power where it was measured, usually, it is 1/2 of max power, where is max power = power when THD+N reached 1%. If that power is not specified, it is cheating again because a vendor may found the particular power level where is THD+N is minimal. Sometimes I saw vendors simply copy/paste specs from parts used in that product. If somebody did see that nice recourse
https://www.audiosciencereview.com/forum/index.php?reviews/ check this out, how often real measurements mismatch vs claimed specs ))