Normally every document section has its own page settings. Page settings include the paper format (width, height), page orientation, margins, and a few other characteristics shared by the section pages. For more information about individual components of page settings please see the Help topic of the File | Page Settings... dialog.
When a document contains multiple sections, you can specify different page settings for each section. For example, you could specify larger page margins for the first section, or the landscape page orientation for the last section of this document.
Note. Some components of page settings are shared by all the document sections. When you toggle "Mirror margins" or "Different odd and even headers/footers", the whole document is affected, ie all the document sections.
The File | Page Settings... dialog allows to apply the required page settings either to the whole document, or to the current section, or to the selected sections, or to the selected text.
Note. The pages belonging to a contiguous sequence of sections separated with continuous section breaks, share the page settings of the first section from this sequence.
When you change page settings through the Atlantis rulers, new settings are applied either to the selected sections (when multiple sections are selected), or to the current section.