When any message is submitted to SMSC, SMSC issue a unique message id against each message. It can be in decimal or in hex format. Subsequently when they provide the DLR, SMSC should send exactly the same message ID in DLR so that iTextPro can match both and update the DLR.
However, there are few SMSC which do not follow this pattern. During submission they issue message id in decimal format , but in DLR they change it to Hexadecimal format. Though the conversion value of both remains same but our iTextPro can’t recognize this. This is the reason, iTextPro extended this feature. You can cross check with your SMSC, if they behave as per expectation, then keep it Decimal, but if otherwise, we have this option to rescue you.