The Microsoft HTML Help (CHM) format uses sets of definitions called "help windows" to configure the appearance, size and behavior of the Windows viewer used to display CHM help files. Help window definitions can also be used to display individual topics in external windows.
Changing the definition of the default help window, which is called Main, adjusts the help viewer features. You can also define additional "secondary" help window types, which can have different features. These help windows are used for opening specific topics in external windows when you link to them. You do this by specifying a different help window type when you define the hyperlink.
Help window settings are only relevant for HTML Help output, where they control the appearance and behavior of the Microsoft CHM viewer.