Comment 5 for bug 734686

I'm trying hard to remember why I added that "per category".

Even trying to construct a hypothetical case, I can't find any reason.
The only thing that would matter would be equal named units with different meanings, like minutes are time and astronomical angle. But that's way of all use cases I could expect.

I think it was just a mere minimal requirement and can be superseeded by a stronger constraint like "for all".