I understand that one may want to preserve the "logical" structure of a book, and let the reading system worry about how to present and interact with the TOC.
For instance, if a book has parts, and chapters inside each part, and then a preface and epilogue, I may want to define the preface and epilogue at the same level as chapters, even if they are not inside a part.
Don't ask me what would be an unambiguous way to display this structure, though
