This is the storage place of a book (collection of pages). You can load it from here, generate a
PDF, let a book printed, edit it or otherwise update its content. Be reminded that when overwriting this book storage page with an updated version of the book, the previously assigned categories are not automatically copied to the recent version: you need to manually copy the categories from the former version.
(Problem has been filed as enhancement request https://bugzilla.wikimedia.org/show_bug.cgi?id=27952)