Catch messages of Procurement Document (PD) layer in SRM
There are lot of cases, where the message source is not the channel or the BO layer in SRM. In this case, put your external break-point to the function BBP_PD_MSG_ADD.
This helps you to identify the right source code part and logic adding this message, based on some criteria. This function adds an individual message to a collection in PD function layer. This message collection is passed layers above, but there you would have difficulties to find the origin of the message.