In Rhinoscript’s definition of UnitSystem it says:
It cannot be both!
It is not both, it is either…
If you don’t supply a unit system, nothing is changed, and it returns (reports) the current unit system.
If you supply a new unit system as an optional argument, then the unit system is changed to the new one, and it returns what the previous unit system was. (it doesn’t return the new unit system, because… you already know that.)
In either case, if it is unsuccessful in determining or changing the unit system (unlikely) it will return Null (None).
Yeah, there is an extra ‘not’ in the second bit, I guess is the problem. I blame copy and paste.
RH-57721 Typo in Help
-Pascal
Ahh… funny how your brain simply ignores that kind of stuff when you are looking at something which you are sure already know the answer to… 
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Oh, I never do that.
-Pascal
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