Online indexes should look similar to print indexes: they should have two levels with indented subentries. Many of the same conceptual guidelines for print indexes apply to online indexes.
The keyword is the term that a user associates with a specific task or set of information. The user types a keyword in the Find or Index box to locate specific information in a document. A keyword can lead to a single topic or to many related topics.
When deciding what keywords to list, consider these categories:
Terms for a novice user of your product
Terms for an advanced user of your product
Common synonyms for words in the topics
Words that describe the topic generally
Words that describe the topic specifically
Words commonly used ...