It’s essentially a sort of rotation. We have 2 GSSR banners per year (New Years and Anniversary), and they seem to follow this pattern
Pool splits: big single pool with everything -> split by Knights/Cavalry -> split by Class
Servants contained: only permanent/story-locked (e.g. non-limited) servants -> limited servants included -> only limited servants
Overall, this has lead to the following progression:
Big pool with only non limited servants (New Year) -> non-limited servants split into Knights/cavalry (1st annivesary) -> non-limited servants split by class (New Year) -> all servants in big pool (2nd anniversary) -> all servants split into Knights/cavalry -> all servants split by class (3rd anniversary; this is the upcoming one on NA) -> only limited servants in big pool (New Years) -> only limited servants split into Knights/cavalry (4th Anniversary) -> only limited servants split by class (New Year) -> ???
It will be interesting to see what JP does for the upcoming 5th Anniversary, as the last New Years banner has exhausted the pattern by which we can predict. It will likely one big pool again, but we can’t say what servants will actually be put into it.