Here's how it works imv:
Ult group = the one group you're most invested in
Stan groups = the handful of groups that are your favorites, but not necessarily your ult. Meaning, you know a lot of things about them - their names, birthdays, what they look like, etc. - but not everything, and you still love the music but not necessarily enough to go to a concert or buy merch. I think
@Jimimis brought up a good point about who can go on your stan list. It doesn't mean at all that you have to list every group you listen to - just the ones you like THE MOST.
"Getting to know better" groups = groups you've recently started really liking but you don't know enough about to be considered a stan of. It doesn't have to be a group that just debuted in like the past year, but it can also be a more well-established group.
Casual groups = every other group you listen to that doesn't fit any of the above categories