I looked at that article and searched the knowledge base as well. I'm not sure this screenshot will help but here is what I was describing when I try to move a notebook so it stacks with another.
But if I want to move Fire and Grass so it stacks under Annual Fire, I cannot unless I create the "new stack" as indicated by the screenshot. If I do this, I have 2 notebooks of the same name, 1 stackable, 1 not.