The sub-navigation, also called the sub-nav, lists pages that fall into specific sections of your site. It appears on the left side of landing and content pages.
Sub-navs are generated automatically when you make one page the child of another. Each of a page's children are included in the list.
If a child page also has children, those pages will appear in the sub-nav when the parent page is opened
The pages generally appear in the order in which they're created. However, you can change the order if you choose.