Mostly it’s a measure of how much oxygen-carrying blood your cardiovascular system can deliver to your muscles. Unless you have some neuromuscular disease, your muscles ability to utilize that oxygen is not a limiting factor.
The CV of repeat testing of VO2max is 2-4%, with the majority of that variability normally being biological, not technological, in nature. However, not all labs/metabolic carts are created equal, so YMMV.
I use the PNOE in my lab. It is only necessary if you are also testing your Resting Metabolic Rate. I allow the athletes to eat a bar or protein shake before the test. If you are just doing a VO2 max you definitely don’t want a full stomach. The last thing you need is your last meal on the floor. lol
I would do it 2nd week after a rest week. Last rest week I didn’t get to ride and first vo2 intervals felt bad