This holds the Distinguished Name of the traffic source object (such as a specific vNIC or uplink port) that this NetflowMonitorRef points at. An administrator uses sourceDn to trace which exact interface a given NetFlow monitor is actually sampling, since the monitor itself is a separate configuration object from the thing it watches. For example, if NetFlow export volume for one monitor looks abnormally low, checking sourceDn confirms whether it is even attached to the busy interface the administrator expected it to be watching.