The idea of custom difficulty names was to follow other difficulty convention names, like [Advanced], [Expert] or [Maniac]. Most of the time it isn't a problem, though there are some maps who use stupid difficulty names that don't resemble any difficulty naming convention... for example, calling the difficulties [Maria], [Joel] and [Daniela] is stupid and confusing because I have no way to know what difficulty is which.
It shouldn't be an obscure difficulty name either, so everyone understands what difficulty it is; for example, let's say a beatmap is of X anime and the strongest character is Y, so the mapper names it [Y]. Those who don't know the anime are left out, wondering what the name means, which is a bit unfair.
Anyone agree?
There was nothing of this written anywhere, so I thought of adding it to the guidelines so there's less confusion.