Although the design of XML focuses on documents, the language is widely used for the representation of arbitrary data structures such as those used in web services.
Several schema systems exist to aid in the definition of XML-based languages, while programmers have developed many application programming interfaces (APIs) to aid the processing of XML data.
The essence of why extensible markup languages are necessary is explained at Markup language (for example, see Markup language § XML) and at Standard Generalized Markup Language.
Hundreds of document formats using XML syntax have been developed, including RSS, Atom, SOAP, SVG, and XHTML. are based on XML and the rich features of the XML schema specification.
XML-based formats have become the default for many office-productivity tools, including Microsoft Office (Office Open XML), Open and Libre Office (Open Document), and Apple's i Work. Apple has an implementation of a registry based on XML. Many of these standards are quite complex and it is not uncommon for a specification to comprise several thousand pages.