The content files contain the help topics, including HTML, images, stylesheets, scripts, samples and other files. These are known as Compiled HTML Help files or CHMs. They make use of the ITSF filesystem format.
CHI, CHW and CHQ files store merged Indexes, ALinks/KLinks and Search information respectfully for merged CHMs and collections of CHMs. They are located in the same folder as the CHM and have the ITSF format, but are usually not compressed, ensuring speedy access.
CHS files store information about the subsets that the user and or help author has created. They are located in the same folder as the CHM and have their own binary format.
The KPD files store information on how to update a CHM. It is used by the KeyUpdate ActiveX control by the KeyWorks.net people. WARNING: This is only BETA technology, so it is NOT guaranteed to be stable.