The idea of rotation its for rotating your activities that your other group may not with to partake on. Example:
You like to play warhammer, you also like to idk play basketball, The 3 or 4 people that are in your main group like both warhammer and basketball, Your fat warhammer friends do not wish to play basketball and your jock basketball friends do not wish to play with figurines, but both of you like to play smash and watch anime or whatever else, So you rotate your friends, sometimes you want to play basketball and these jocks will do it, sometimes you'll play warhammer and the fat guys will do it, but your core friend group doesn't change. That goes for every thing you want to do, If you started watching extreme mountain biking videos and wanted to do it, it its unlikely that people on the previous groups would do it. So you just make some extreme mountain bike friends instead. The rotation its how you spend your time rather than the people. The Core group its also cultivated within years, you'll probably start with 0(middle school) and hopefully end up with 4 or 5 when you are older and making real true friends instead of acquaintances its probably out of the picture(middleage).