Overview
You are curious what the “err” field value means and what are some possible values for this field. You found this field in Appendix B of the SMPP v3.4.
Solution
Please refer to the HUB Operator Manual, Appendix B.3 SMPP Notification Text Error Codes.
There you can find the table that maps HUB internal MXP error codes to 'Err' field values in the SMPP Notification text, which is then sent to the originating application in the 'short_message' field of the Deliver_SM carrying the delivery receipt.