Used to create a site map file for search engines. The file informs search engines that you have pages available for crawling, and can be submitted to Google or Yahoo.
The site map file is created when you add your web address as the Base URL in the book's Web tab. You can add the URL to the book or the book template. When you publish. the book the file is created and added to your publishing folder.
The WebBaseURL element corresponds to the "Base URL" field on the HTML tab of the Book object.