Note that A-keywords are a Microsoft help technology that is only supported in the Microsoft HTML Help (CHM) format. A-keywords are irrelevant in all other output formats, including WebHelp.
A-keywords, also known as "A-link keywords", are only used in Microsoft's HTML Help (CHM) format. They are quite similar to normal keywords but they not displayed in the index, they are always "hidden". What use is a keyword that isn't displayed anywhere? There are two major uses for A-keywords: To create "See also" lists of related topics and to create links between help files in modular help systems.
See About A-Keywords for background information.
About A-Keywords (Reference)