The following information is only relevant for the Microsoft HTML Help CHM format.
Help windows define the features of the Microsoft help viewer for HTML Help and are also used to display individual topics in external windows. When you define a hyperlink to a topic you can include a setting that specifies a secondary help window for opening the topic. This allows you to create links that open topics that would normally open in the TOC in external windows.
See Using help windows for more details.
Help Windows (Project Configuration)
Help Windows (Reference)