The Test Request <1> message forces a heartbeat from the opposing application. The Test Request <1> message checks sequence numbers or verifies communication line status. The opposite application responds to the Test Request <1> with a Heartbeat <0> containing the TestReqID <112>.
The TestReqID <112> verifies that the opposite application is generating the heartbeat as the result of Test Request <1> and not a normal timeout. The opposite application includes the TestReqID <112> in the resulting Heartbeat <0>. Any string can be used as the TestReqID <112> (one suggestion is to use a timestamp string).