Pages for Mac: section
A document section is a portion of a word-processing document, such as a chapter, to which you can apply formatting different from the rest of the document. For example, a section can have different page numbering, background images, headers and footers, and table of contents.
A word-processing document is all one section unless you specifically add more sections to it.
When you select a page in Page Thumbnails view (on the left side of the Pages window), a blue highlight appears behind all the pages in the same section.