How would we classify this? (Games that are "volumes")

[TeridaxXD001]

https://www.project-imas.com/wiki/TH...ravure_For_You!

There are two series of photography-centric games, THE iDOLM@STER: Gravure For You! and THE iDOLM@STER Cinderella Girls: Gravure For You!. Each series contains 9 games, each labeled as a "volume." There are two ways I could see doing this: 18 entries, one for each volume, or two entries, one for each series. The differences between the games are different costumes, locations and musical tracks, but the characters and voice casts are the same.

On the other hand, these are not add-ons or DLC packs: each volume is a self-contained game that comes by itself on its own disc. So I'm having a hard time coming up with how I'd put together a request for this.