This tab allows you to view and edit the XML source code of the current topic (Professional and Server versions only). You will almost never need to use this when working on projects.
Although XML looks similar to HTML at first glance it is actually totally different. It is also 100% strict and does not tolerate even the smallest syntax error. If you make any edit that does not conform to the Help+Manual XML schema, the editor will silently delete your edits when you return to the main topic editor or move to another topic.
See XML and XML Editing for more information on this subject.