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The Extensible Markup Language (XML) is a general-purpose markup language—in some ways, much like a family of markup languages. Its primary purpose is to facilitate the sharing of structured data across different information systems. It is used throughout EServerCollections both to encode documents and to serialize data.

It started as a simplified subset of the Standard Generalized Markup Language ([SGML]?), and is designed to be relatively human-legible. Moreover, XML is sometimes used as the specification language for such application languages. By adding semantic constraints, specific applications, such as [XHTML]? and [RSS]?, can be implemented in XML. The EServer uses XML extensively.