They can create duchy and kingdom level titles if your realm is too big and one heir inherits enough stuff that's not de jure a part of your highest title.
Confederate Partition means that if any heirs can make a new title when they inherit their realms on succession, they do so automatically. Turns out that CK2 Gavelkind is CK3 Partition. I'd been operating under the assumption that Confederate Partition was just the new term for Gavelkind, until I finally got to Early Medieval era with Bengali culture doing a side playthrough in Pala and looked over my options to find that there's just a normal Partition succession. well, I think I'm probably not gonna do many more 867 starts.
So, I finally did some in-depth reading on succession laws and.