It doesn't matter as long as the file you send to Apple is .epub. What are you using to write your book? It would be best to use an app which has export to epub built in.
I am using iBA to create the book.
iBA cannot create epub. So you would have to redo or copy/paste your book into another app. The Apple app for create epub is Pages.
Books created by iBA can only be read on an iPad.