An index is often the readers' primary information retrieval device. When readers search for a particular topic and find it referenced in an index, they are assured that the topic is covered in the document and need look no further. When, on the other hand, readers don't find it in an index, they may decide that the topic is not covered in that document and look elsewhere.
In surveys of readers of technical documents, many complain about the inability to find the information they are seeking. An index goes a long way toward solving the problem of locating information.
This chapter describes how to prepare an index for a technical manual.
Note - This chapter makes recommendations, but there are many other acceptable styles and theories of indexing.
According to The Chicago Manual of Style:
A good index records every pertinent statement made within the body of the text. The key word here is pertinent. The subject matter and purpose of the book determine which statements are pertinent and which statements are peripheral. An index should be considerably more than an expanded table of contents and considerably less than a concordance of words and phrases.