The WSDL description language was designed to enable composition from multiple sources and to enable reusability of individual WSDL artifacts. For example, multiple service may implement the same interface (portType) or exchange messages that conform to the same schema. Therefore you can define each component of a service's definition in separate WSDL documents. Message structures should always be defined in a schema, which may be inline in the WSDL document, but often is imported from a separate file (using the <xsd:import> statement). When assembling the WSDL for a specific service, you use the <wsdl:import> statement to import the different WSDL files.
This was first published in April 2005