An authority resource is one you would cite in court or legal documents because its authority is undisputed.
Legal research resources are often divided into primary and secondary authorities. Primary authority is the law, while secondary authority is often characterized as commentary on the law. Where does a case law digest, an index, or table of cases fall? They aren't commentary on the law nor are they the law itself. They are not authority at all. They are finding aids. So legal research resources can be divided into three categories:
Only Primary Authority can be further subdivided into mandatory and persuasive authority.