An associated source definition is specific to the message data format and defines the message data field in the WebSphere MQ message. An associated source definition can be a flat file or VSAM source definition. If the WebSphere MQ message contains flat file or COBOL message data, you need to use an associated source definition with the MQSeries source definition to read and transform the message data. You join the MQSeries source definition with the associated source in the MQ Source Qualifier.