I wonder if, in the U.S., the whole notion of systematized ratings is less common than the practice of just assigning an informal description to get the point across. Words like “easy”, “moderate”, and “difficult” get posted on a sign and therefore mappers historically tagged them in name in parentheses, but they don’t necessarily form a system that you could name. It could be something like a freeform difficulty key, or maybe difficulty:ref, by analogy with a similarly intentionally heterogeneous key, level:ref.
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