About the Hidden Information Type
You can define a Hidden information type to identify or group topics when a program is started, before a user sees them. You select which topics will appear based on the elements of the program that are active. Hidden information types are never seen by users; they are instead controlled by the program.
This feature allows programs, through the HTML Help API or through hosting the HTML Help ActiveX control, to determine what information is presented to the user. The Hidden information type attribute also determines which URLs should be linked to when they are associated with an information type.