The 2nd generation is Wonder Girls Tell Me.
The 3rd generation is Twice Cheer Up
The 4th generation is Espanola
Based on big hit songs, if you debuted around the same time and made your name known, you are grouped into the same generation.
If it was famous before then, it is grouped into the previous generation.
The groups that debuted these days are called the 5th generation.
If they don’t come out with a generation-dividing hit song quickly, I think these groups will all be grouped into the 4.5 generation over time.