Since HTML was defined for designing web pages, XML is strong in describing and exchanging structured information. XML is not just a language itself, but rather a meta-language, a framework for defining other languages. Next, like a word in natural language a meaning is assigned to the elements that then can serve as markup. XML describes how markups are to be built and assigned to a meaning. Thus, all XML markups follow the same syntax.
Markups are the first steps for structured information exchange.
XML doesn't come default with markup for purchase orders or purchase requisition. Everybody can write them themselves. This feature frees applications programmers from the constraints of predefined structures and let them structure the data in an XML document any way they like it. While humans may realize that those are most likely meant to be the same thing, machines will definitely fail. For XML to be widely used, the e-business community must agree upon a standard set of tag names and associate a commonly understood meaning to it.