It's not like HTML except for being XML which looks a lot like SGML.
XML does not only look a lot like SGML, it is a proper subset of SGML:
XML is an application profile or restricted form of SGML, the Standard Generalized Markup Language [ISO 8879]. By construction, XML documents are conforming SGML documents.
Unlike HTML, btw, which started out as a loose subset of SGML but has abandoned the goal of SGML compatibility in favour of conceptual simplicity.
It's significantly more limited and more specific to the task than HTML. HTML comes with a lot of baggage that makes it unsuitable for a really specific use case.