XML Schema Complete Reference, The

3.6. XML Schema and Namespaces

You associate an XML Schema with a namespace by using the targetNamespace attribute. A single XML Schema document can support at most one XML Namespace. Also, you can compose an XML Schema from multiple XML Schema documents that all share the same target namespace. You can also import elements and types from other namespaces in your schema by using the import element. By importing schemas from other namespaces, you can compose schemas that combine multiple vocabularies. Chapter 7 covers use of the import element.

In XML schemas, structure types are identified by QNames. This means they either need to be prefixed, there needs to be an in scope default namespace, or there must be no target namespace for the schema. ...