The pages in the output document are actually created when the XSL processor processes fo:page-sequence objects. A page sequence consists of a run of pages that share the same characteristics, such as a chapter in a book.
Each fo:page-sequence object references either a fo:page-sequence-master or a page master, and the actual layout of the pages is specified by those masters. You can get fairly involved here, creating sequences in which the page numbering alternates from side to side on the page as when you're creating pages for a book. Here are the properties that apply to the fo:page-sequence object: