The mqttSend block sends to the MQTT test broker test.mosquitto.org at topic "mdd-example". The mqttReceive block subscribes to the same broker at the same topic "mdd-example". Consequently, the mqttReceive block receives what the mqttSend block sends.
Note: There is no causality between the mqttSend block and the mqttReceive block. Therefore the execution order of the blocks is not determined. Additionally, the mqttReceive block starts an own receiving thread, so that the time the data was received is not equal to the time the external function within the mqttReceive block was called. This indeterminism may also show up in the plots.