Uniqueness and Document-Oriented Issues

ID, IDREF, and IDREFS Like XML 1.0 DTDs, XML Schemas provide the ID type for identifiers that must be unique within a document, IDREF for values that must match one of those identifiers, and IDREFS for cases where more than one value might be referenced.
Keys and keyrefs XML schemas add the concept of keys to the ID/IDREF mix, using XPath information to add requirements to documents. (In some ways this feels like schematron. These tools allow schema developers to add requirements to a document on a for-each key this corresponding keyref basis.
Unique The unique element allows schema designers to create selectors with a subset of XPath identifying a particular region in a document and then specify a particular field that must be unique within that region or the document as a whole.