MDC stands for Mapped Diagnostic Context. Mapped Diagnostic Context enriched the logs by providing more information about the event in the logs. By default, Mule logs two entries: processor which shows the location of current events, events which shows the correlation Id of the event.
Mule Runtime 4.4 introduced Tracing module and enables you to add more information to the logs by by adding, removing, and clearing variables from the logging context for a given Mule event.