The issue of categorizing Japanese TV dramas

Japanese TV dramas often have special episodes or side stories with a certain distance between them.

Like https://www.imdb.com/title/tt39709674/

It includes the main series, which is the first season.

Special episodes, usually at least 30 minutes long, are generally additional stories; Japanese dramas typically don't use multi-season production methods.

Bonus episodes (Spin-off) are short films that are released alongside the main series.

First season: use "S1:EX"

Special episodes: use "Unknow"

Bonus episodes (Spin-off): Is there a suitable genre that could be included in "Unknown" and interspersed with special episodes?