Sidebar Navigation
Use this pattern to place a dynamic sidebar menu onto pages that aren’t included in the main navigation at the top of your site.
The Sidebar Navigation auto-populates based on the nesting structure in your site’s Main Navigation Menu: pages at the first two levels appear in the top menu on all pages, and pages at level three and below will continue into the Sidebar Navigation pattern if you place it at the top of a page.
Consistency is critical for navigation, so use it at the top of most pages that aren’t in your main menu. Exceptions that might be nested at level three or below but don’t tend to need sidebar navigation include collections many of like items ( e.g. a news archive or employee directory) and user task endpoints like fillable full-page forms.