I will just quickly say that this would be like a Table Of Contents or list of Categories or Chapters or Subjects within which will possibly be pages or Sub Index that would have Topics containing pages. This is a "tree" structure for organizing pages within a group so that they can be related to each other not only by having internal links from Parent to Child pages. There is also the "Linked From" page list that is generated by clicking on this link, which is located in the footer - but not functioning since we migrated to V.2.
Index Pages? are related to Wiki Trails and so there are also other kinds of indexes than the Chapter and Topic Indexes - for example we can have cross reference indexes and "referred from" indexes as well as Page Maps.