XML is the industry standard for precisely representing data from virtually any source, stored in virtually any format. In appearance, it is similar to HTML, but while HTML explains a document in terms of how it should display data in a Web browser, XML is the data (or more precisely, the data from an application represented as XML).

This data can be from an application screen, sometimes called a screen scraping, it can be the output from a database, or it can be an application executed using processing instructions that run Siebel eScript, for example.

There are also technologies that explain XML documents. These are known as metadata because the data within these documents is used to describe and format the information in an XML document. Examples of metadata documents include XSDs (XML Schema Definitions), DTDs (Document Type Definitions), and XDRs (XML Data Reduced), which are supported by Siebel applications.