Users are presented with two fields as it relates to section assignment in the CMS: Primary Section and Additional Sections.
The Primary Section defines the canonical home of the content item. This is used to construct the URL of the content item (in most cases, unless you’re using date based URLs), for ad serving, for breadcrumbs, and for section labels. This is required in order for a content item to publish.
Additional Sections define other areas of the site where the content item may live. This is not used for ad serving, breadcrumbs, or section labels. These are essentially alternative homes for the content across the site.