Measurement units can, possibly, describe some agency that already has other units including SI units, but be an island unto themselves regarding relative accuracy better than the accuracy with which their conversion factors to the other units are known.
For example, early in the history of the angstrom length unit, its value was more precisely known than the length of the metal bar that defined the meter at the time. I think this was because it entered into angle determinations in X-ray crystallography.
For another example, the astronomical unit was originally more precisely defined within its own usage and its usage in defining the parsec, relative to how precisely the length of an astronomical unit was known in any other distance units.
I think the electron volt may have ben like this too. It was a unit of energy, but to convert it to other units of energy you have to know the electron’s charge in coulombs, and I’m not sure that was so accurately known. I could be wrong on that one, but you get the idea. And it might be argued that it isn’t a unit of energy, but, rather, an experimental result, just as one might argue an inch of water column isn’t a pressure unit but rather a description of an experimental outcome. Messy, maybe.
So, you can convert feet and inches and meters and miles with perfect accuracy and trivial effort, but there are these other units out there that are more strongly linked together within their own usage than they are to the main units used elsewhere.
Is there a special name for such a unit?