Links and secondary windows
Help windows define the features of the Microsoft help viewer for HTML Help and are also used to display individual topics in external windows. Help windows are a Microsoft technology that is only relevant for HTML Help output.
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.
1.Define at least one secondary help window type.
2.In the window's HTML Help Options tab select the Links to secondary help windows open a new help window option. (This allows external windows in HTML Help.)
3.You will probably also want to use the other options in the HTML Help Options tab to switch off the external windows' control buttons and navigation panel.
Opening target topics in an external window:
Create a hyperlink to a topic, open the Hyperlink dialog (double-click on the link) and select the secondary window type in the Window: field, then click on OK.
Help Windows (Project Configuration)
Help Windows (Reference)