The following links are to the documentation created and maintained by OCLC, vendor of WorldCat Discovery aka "Encompass Search."
Note that OCLC maintains three different search platforms: Connexion (our "back-end); FirstSearch; and WorldShare / WorldCat Discovery. Many of the indexes are the same across the platforms, but many differ considerably (or are unavailable) in certain platforms.
Before using a certain Search index, be sure that it is available in WorldShare and WorldCat Discovery.