You can tack on “unless more rigorously defined by the specific game system” someplace, sure.

Who decides?  Well, again, it all depends on the specific system.  US has a gamemaster and I’d bet dollars to donuts that that GM gets that authority, either explicitly in the text or by convention because “that’s what the GM does”.

Beyond that, we’ve got all manner of tested solutions.  I think the best one that works without explicit authority is “the most discerning player” decides.