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Note: The following is an excerpt from the official NIST catalog. It is provided here as a connivence and is not authoritative. Refer to the original document as the authoritative reference.

The purpose of XML Schema Definition Language: Structures is to define the nature of XSD schemas and their component parts, provide an inventory of XML markup constructs with which to represent schemas, and define the application of schemas to XML documents.

The purpose of an XSD schema is to define and describe a class of XML documents by using schema components to constrain and document the meaning, usage and relationships of their constituent parts: datatypes, elements and their content and attributes and their values. Schemas can also provide for the specification of additional document information, such as normalization and defaulting of attribute and element values. Schemas have facilities for self-documentation. Thus, XML Schema Definition Language: Structures can be used to define, describe and catalogue XML vocabularies for classes of XML documents.

Any application that consumes well-formed XML can use the formalism defined here to express syntactic, structural and value constraints applicable to its document instances. The XSD formalism allows a useful level of constraint checking to be described and implemented for a wide spectrum of XML applications. However, the language defined by this specification does not attempt to provide all the facilities that might be needed by applications. Some applications will require constraint capabilities not expressible in this language, and so will need to perform their own additional validations.