This rule is totally out-to-date and should be nuked. Nowadays converted CtB diffs (talking about the highest difficulty in an osu!standard beatmap) are a total disaster in most of the cases. Either they're boring to play or pretty uncorfortable or even "impossible" to FC due to the overuse of slider-jumps and other kind of spreaded 1/4 sliders. The rule was quite okay 1-2 years ago, but it's time to evolve and reword this rule:
- Having just one CtB difficulty is allowed in an osu!standard mapset because standard difficulties are converted correctly to CtB. Thus, in Taiko or osu!mania mapsets you need at least 2 CtB difficulties since Taiko and osu!mania can't be converted to CtB.
...into something like this:
According to this rule change, the allowed cases would be the following ones (correct me if I'm wrong):
- Having just one CtB difficulty is allowed in an osu!standard mapset if it isn't an Overdose level (or an Extra icon CtB difficulty), because standard difficulties are converted correctly to CtB. Thus, in Taiko or osu!mania mapsets you need at least 2 CtB difficulties since Taiko and osu!mania can't be converted to CtB.
- Mapset with an osu!standard spread + mode specific difficulties: Not necessary to make a full spread of them. Having difficulties from the "Hard" level and on would be enough. e.g. std ENHI + Taiko HI / std ENHIX + CtB HIX.
- Mapset with specific difficulties only: A full spread of each participant game mode is required, applying also the rule "The mapset must have a well-designed spread of difficulties, containing at least an Easy or a Normal difficulty". e.g. 7K ENHI + CtB NHI