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.