A document section is a portion of a document, such as a chapter, to which you can apply different formatting from the rest of the document. For example, a section can have different page numbering, background images, headers and footers, and even tables of contents.
The control for creating a new section is at the bottom of the Section pane of the Document Setup inspector (click Document
, then click the Section button).
In thumbnail view, a yellow box appears around the pages in a section.