For the most part, MSMQ messages refer to the messages your application uses to send data. However, there are also several MSMQ-generated messages (for example, acknowledgment and report messages) that provide additional functionality.
Your application can use any message property to identify a message, including a message's MSMQ-generated identifier, application-defined identifiers, label, class, body, and so forth.
Each message, whether it is sent by an application or generated by MSMQ, receives an MSMQ-generated message identifier when it is sent.
The two application-defined identifiers are the correlation identifier and application-specific identifier. By using these identifiers, you can create your own method for identifying messages.
For information on how to set the correlation identifier and application-specific identifier, see PROPID_M_CORRELATIONID and PROPID_M_APPSPECIFIC (for applications using ActiveX components, see CorrelationId and AppSpecific).