XML is only relevant at all if you are using the Professional or Server version of Help+Manual. The Basic version does not have an XML editing tab in the Editor window and can only save in the compressed, single-file .hmxz format in which you do not have access to the XML source files of your project. (The Basic version can edit uncompressed XML projects however, so that it can be used as a translation editor.)
Working directly with the XML source code is something for advanced users only. You do not have to know anything at all about XML to work with Help+Manual.