Categories in iBooks

iPad OS Ver 9.2.1 iBooks Ver 4.6

I am successfully getting to grips with Collections in iBooks but I cannot see any point to Categories. I have the following questions.

1) What can Categories do that Collections cannot? As far as I can see they both simply allow books to be grouped on the basis of single words or phrases.

2) Whereas Collections are flexible in that they can be created and deleted, books can be moved to and from them and they can be renamed, there is no obvious flexibility whatsoever to Categories - you are stuck with whatever iBooks decides is best for you. If there is some way of editing Categories I would appreciate knowing how to do it.

3) IBooks shows no rhyme or reason on what it does with its imposed Categories. For example it has insisted on putting Dostoevesky's Crime and Punishment under Literary Criticism. Really? That's news to me and, I imagine, thousands of other people. And, as far as I can see, there is no way of correcting this nonsense.

4) I've added a dozen or so books and for reasons best known to iBooks most haven't been given any Category. Why only a few and not all of them? Also a small number were placed in Books. The remainder were also books, so what is the subtle distinction that iBooks makes?

In case it is relevant, I transferred the books to my iPad by means of the wifi Content Server on Calibre.