KiteLocation
Where a page sits in the CURRENT layout: chapter, then page inside it.
A document is a list of chapters, and each chapter is a list of pages. A PDF is one chapter; an EPUB is one chapter per spine item. Chapters exist because a reflowable book can lay out one of them without the others, so a reader can open at chapter 20 without paginating chapters 0 to 19 first.
This is coordinates, not identity. Change the font size and the same words are at a different KiteLocation. To remember a reading position across a re-flow, save a KiteBookmark instead.