How difficult work?

Level difficulty is automatically calculated from the clear count and the death count of the level (=Clear rate).
Clear rate = Cleared / (Cleared + Failed) x 100
A level that has never been cleared by anyone has no clear rate, so it is treated as incompleted and shown with an unknown difficulty icon.

Exact value

Level	Name	Clear rate	Tooltip in game
1	EASY	50 ~ 100 %	`ClearRate 50~100%`
2	NORMAL	20 ~ 50 %	`ClearRate 20~50%`
3	HARD	5 ~ 20 %	`ClearRate 5~20%`
4	EXPERT	1 ~ 5 %	`ClearRate 1~5%`
5	HELL	0.5 ~ 1 %	`ClearRate 0.5~1%`
6	HELL+	0 ~ 0.5 %	`ClearRate 0~0.5%`
7	(incompleted)	never cleared	-

A boundary value belongs to the harder level. For example a clear rate of exactly 20 % is HARD, not NORMAL.
Level 1 ~ 6 each have their own icon. It is used by the stage list, the stage detail panel and the difficulty filter.
Level 7 (incompleted) means nobody has cleared the level yet. It has no icon of its own.
Independently of the value above, a level that has almost no play record (cleared 1 time or less, and no death recorded) is drawn with the unknown difficulty icon.
The names EASY ~ HELL+ are the same names Challenge mode uses for its six difficulties.