From XML 4.0 SDK,

Frequently Asked Questions about Schemas
The following list includes frequently asked questions about schemas.

What is the difference between a declaration and a definition?

What is the difference between XML Schema (XSD) and XDR schema?

What is the difference between a declaration and a definition?
Declarations describe the content models (structure) of elements and 
attributes inside XML document instances. Definitions create new data types 
(simple and complex types).

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What is the difference between XML Schema (XSD) and XDR schema?
The XML Schema definition language (XSD) is the current World Wide Web 
Consortium (W3C) specification for XML schemas. XML-Data Reduced (XDR) 
schema is an interim schema language offered to developers while the W3C 
worked on a draft implementation. MSXML versions 4.0 and later support both 
XML Schema (XSD) and XDR schema.





Comparing Schema Languages
This section compares schema languages to help you make decisions about 
implementing schemas into your XML applications.

Document Type Definitions (DTDs)
  a.. DTDs specify content models for a set of XML documents.
  b.. DTDs keep XML documents consistent.
  c.. DTDs can be shared.
  d.. DTDs provide a formal and complete definition of an XML vocabulary.
  e.. DTDs are restricted to one DTD for each XML document.
Schemas
Like a DTD, a schema specifies the structure or model of a particular set of 
documents. Following are the benefits of using a schema language to describe 
XML document structure:

  a.. Schemas use XML syntax.
  b.. Schemas can be parsed by an XML parser.
  c.. Schemas provide support for data types; the type of data contained in 
an element can be specified.
  d.. Schemas allow global elements (elements that must be used the same way 
throughout the XML document) and local elements (elements that have a 
specific meaning in context).
  e.. Schemas offer data type support (integer, string, Boolean), and the 
ability to create other data types.
Support for XDR schemas
The following products support XML-Data Reduced (XDR) schemas:

  a.. Microsoft¢ç BizTalk¢â Server
  b.. Microsoft SQL Server¢â 2000
  c.. Microsoft Office 2000
  d.. Microsoft Internet Explorer version 5.0 and later
XDR schemas also have tools support in the following product:

  a.. Extensibility's XML Authority
XSD Schemas
XML Schema definition language (XSD) schemas provide support for data types 
and for defining structures. This support allows you to create robust XML 
Schemas and validate XML instance documents against these schemas.


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"ad" <ad@w...> wrote in message 
news:OaaXfyxZFHA.3976@T......
> There are three ValidationType : DTD, XSD AND XDR
> What is the difference between them?
>
>