DTDs provide the most basic standard level of validation available to XML. DTDs provide simple structure and string-based content validation; however, they do not provide data typing functionality. DTDs also use a legacy non-XML format inherited from Standard Generalized Markup Language (SGML). DTDs have the advantage of having been part of the XML recommendation since the beginning, so nearly all XML parsers support them to some degree.
XDR schemas represent Microsoft's first attempt at implementing an early working draft of the W3C XML Schema recommendation. XDR functionality is Microsoft-specific, and you won't find it implemented widely on other platforms. XDR functionality for constraining XML structure and typing content has been superseded by the official W3C XML Schema recommendation, and support for XDR is provided for
backward-compatibility reasons. You normally won't use XDR schemas for new functionality, though you might run into them while supporting legacy applications.
XDR schemas represent Microsoft's first attempt at implementing an early working draft of the W3C XML Schema recommendation. XDR functionality is Microsoft-specific, and you won't find it implemented widely on other platforms. XDR functionality for constraining XML structure and typing content has been superseded by the official W3C XML Schema recommendation, and support for XDR is provided for
backward-compatibility reasons. You normally won't use XDR schemas for new functionality, though you might run into them while supporting legacy applications.