side was originally used in tapes, and it is divided into SIDE A and side B. The main song of the album is usually placed in side A.. After the appearance of CD records, because all the songs are on one SIDE, the statement of side is not as common as before. Later, people often use SIDE A and SIDE B to describe the two different natures of the same thing or person. In recent years, in order to express different musical styles, many bands like to attach some music with different styles to the same album. These songs are usually put on another disc, and the previous name is B SIDE. As for side D, you can refer to the previous album G SIDE of GORILLAZ (this album is basically a remix of songs in GORILLAZ), which is a variant of B SIDE, and G is the abbreviation of the name of the first album of Gorillaz. Moreover, most of the songs in D Side are remixes of Demon Day, so it is likely that Gorillaz will soon release a remix version named after the abbreviation of the album name plus Side for each new album.

It's the first time I've seen Side as a title. Hehe, I don't know about Japanese bands. Maybe the Roman spellings of some people's names in them are D and K. The usage of SIDE I said is only universal, just like many scandals started with Watergate. If you want to know the meaning of the letters after SIDE, you must have a deep understanding of the band.