The Popup tab is only available when the HTML Jump Type property is set to Standard Popup, and defines how the links in the Hypertext Links tab will be converted into HTML Help pop-ups in the published web outputs.
Only topic links are converted into HTML Help pop-ups. Links to external files and Internet URLs are ignored. The Topic Heading, all formatting, and any graphics are ignored, and only the plain text of the topic text is inserted as a HTML Help popup.
