Section 5 covers more advanced features, including a mechanism for specifying uniqueness among attributes and elements, a mechanism for using types across namespaces, a mechanism for extending types based on namespaces, and a description of how documents are checked for conformance. In addition to the sections just described, the primer contains a number of appendices that provide detailed reference information on simple types and a regular expression language. Structures specifies the XML Schema definition language, which offers facilities for describing the structure and constraining the contents of XML 1.0 documents, including those which exploit the XML Namespace facility. Structures is to define the nature of XML 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. Schemas may 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.
Structures formalism to express syntactic, structural and value constraints applicable to its document instances. Structures 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 any application. Some applications may require constraint capabilities not expressible in this language, and so may need to perform their own additional validations. Datatypes is part 2 of the specification of the XML Schema language.