A more thorough explanation:

An index of authorities is a list of sources that are cited in a book or legal brief. It is usually arranged alphabetically and may have subcategories for cases, statutes, and treatises. This index helps readers locate the sources that were used to support the arguments or ideas presented in the book or brief.

• Case: Roe v. Wade, 410 U.S. 113 (1973)
• Statute: Americans with Disabilities Act, 42 U.S.C. § 12101 et seq.
• Treatise: Black's Law Dictionary (10th ed. 2014)

In the example above, the index of authorities includes a case (Roe v. Wade), a statute (Americans with Disabilities Act), and a treatise (Black's Law Dictionary). Each source is listed with its proper citation format, which includes the title, author, edition, and year of publication. This allows readers to easily find and reference the sources that were used in the book or brief.