The subject is starting from http://osu.ppy.sh/forum/p/4725284 where I misinterpreted a sentence on RC:
The meaning of "clearly indicating their level of difficulty" is ambiguous. I've all been interpreting it as they need to indicate each diff's "absolute" difficulty, but actually it's working as a "relative" difficulty between other diffs, as mapsets like https://osu.ppy.sh/s/340399 are still rankable. I also have seen many other people complaining "Why should we always use strict diff names except the hardest one?" since they were misunderstanding the rule too. So this rule needs a clarification.Ranking Criteria wrote:
A difficulty's name must indicate its level of difficulty, with the exception of the hardest level of difficulty in a set. The mapset's hardest difficulty may use an appropriate custom difficulty name, unrelated to a username. Mapsets may also use a complete set of custom difficulty names that clearly indicate their level of difficulty to the player.