So, this is in the Furion's Halo series, and Billy is our current protagonist in this playlist. Generation Crystal is actually the first generation to ever live on Catters, Billy, Billy's friends, anyone born the same year or 10 years away from when Billy was born counts as Generation Crystal, and that was the only generation to come to Catters because these are normal Earth cats, not mutated yet to live to an average of 70-90 years of life.

Where the music fits Billy's timeline:
Furion's Halo: Billy and Felisha's goodbye (Zeeblue Eurphia theme) - This is when Billy says goodbye to her bestie Felisha.
Furion's Halo: Catters Theme - This is when Billy discovers Catters and starts helping cats construct their homes. They want their homes to look like homes at Earth, so they try their best and create something very similar. At the end of this song, the new cats of Catters ask Billy if she'd want to be their queen. And Billy is of course surprised, but gratefully accepts the offer.