In Flare's web targets, you can prevent people from loading an external website in the topic pane by appending a foreign URL at the end of your output's URL. You might use this feature to prevent someone from appending a malicious website to the end of your normal URL.
This option does not prevent external links that are inserted in a topic from opening (e.g., a hyperlink on some text that is pointing to http://madcapsoftware.com will still open that website). Those types of links point directly to the external URL and will not append to your Help system's URL.
However, this option does prevent external websites from being loaded from the table of contents (TOC) or a browse sequence. For example, if you have a TOC node that points to www.madcapsoftware.com, it will not load with this option enabled since that node creates a hash tag followed by the linked URL (e.g., Path/To/HTML5/Default.htm#http://www.madcapsoftware.com). If such a link exists on a TOC or browse sequence node, it will simply open the starting topic in your output when clicked. Therefore, if you want to use the option to prevent external URLs, you should also avoid putting external links on TOC or browse sequence entries.