What is the sent Message Reference valude?

I am creating an application that uses the SMS service.
When application send an SMS, ADL_SMS_EVENT_SENDING_MR event occur in SmsCtrlHandler() that I specify in the adl_smsSubscribe().
According to “ADL_User_Guide.pdf”, at this time, Nb parameter of SmsCtrlHandler() contains the sent Message Reference valude.
What is the sent Message Reference valude?

value

It’s the same that you get when you do it “manually” using AT+CMGS