Unlike anywhere else in the world of skiing or snowboarding, and according to wikipedia.org, Japan has a very simple ski piste difficulty rating system. In general, slopes are grouped into the following categories to the right, namely, beginner, intermediate an expert. Furthermore, there is no agreed definition of each difficulty. In other words, the three difficulties are relative to one another at each resort. By and large, you should expect them to be similar but there is no guarantee that they will be – caution is needed.
While Japan has three difficulty categories, often the data in OSM is divided into 6 categories (7 if you count “not rated” as a category).
On the main index screen, located here, freshsnow.jp reduces the 6 category to just three although the map continues to display as 6 categories where they do exist. This is mainly done to help you search for the type of mountain you desire by 3 categories. On the individual mountain pages, for example Niseko, they are broken down into 6 categories.