There are dozens of indexes from which to choose. A few indexes are general and include a wide variety of subjects.
Most indexes focus in a particular subject or disciplinary area.
Let's compare choosing radio stations in Chico with choosing indexes to illustrate this concept. Suppose you just moved to Chico and you are setting the radio buttons in your car. You need to know what the different stations have to offer. Each radio station has a specific range of offerings. Some have only music, while others also have talk shows or contests.