Some Japanese bands make about 3 or 5 singles a year, and these singles comprise the following:
1 - The song itself, the B-side, and any instrumental side.
(Ai Otsuka, Morning Musume, Bonnie Pink...)
2 - The song itself, the B-Side, and any remix...
(MISIA, M-Flo, CHEMISTRY...)
3 - The song itself, The B-Side, and any live version of the song
(TAKUI, Dir en Grey...)
The singles are then compiled into a single album. However, some B-sides of the singles are not included in the main album itself. That's one of the major selling points of the single...