BS7671 expressly sets out the conditions pertaining to the ratings.
No doubt the NEC does so too and conductor ratings are based on those conditions.
Now, if the code gives a conversion to a metric size, say 300 kcmil to 201 mm2 which is a larger cross-sectional area, does it specify different operating conditions for metric?
Even if it does, I'm not sure what you would gain in knowing that 300kcmil is rated at the same current as 201 mm2 - it isn't a standard metric size. It falls between 185 and 240.
Given the degree of precision, 201 seems like it might be the result of a calculation. If it is, I'd be curious to know what it is.