Once a book or course has been compiled (i.e., added to the Download Center with PDFs and LMS common cartridge files available), the book will have a Front Matter and Back Matter automatically generated. The Back Mater will have a "Index" page that is is a dynamic index that is constructed by the meta-tags of the pages of the book.
For example, the index for Wakim and Grewal's Human Biology text can be access here:
You can see the term "abiotic factors" in the index at the top.
Section 24.2 is automatically added to the index under "abiotic factors" since that term was manually added to that page's Tags (in Page settigs).
Since there are seven other meta-tags indicated for this page, it will be reflected multiple times in the index.