XML has NO predefined tags. You are free to write whatever you would like inside an XML file. If you see any tags inside an XML file like the one I have shoved above, they have not been defined in any XML standard. Those are tags I created/invented myself. Actually, they are made up of my object inside a project.
If you are familiar with HTML you already know that HTML requires som predefined tags like p, h1, h2, table, div, etc… With XML, you are the author and must define both the tags and document structure. There are no limits or rules.