Sector Index A-Z is used to list content in an alphabetical index - this is the Sector A-Z by Title view, where index items are sorted by their title (see Sector A-Z by Topic for ordering by topic). You can search the index items by keyword as well as filter them by category.
The Sector A-Z by Title view is a Search API view run from a full-text Search API index. This text is added via a 'Global: Text area (Global: Text area)'.