Message Default and Message Processing 
Use
A message default is made in the application transaction (for example purchase order entry). You can change the proposed message manually (edit) before the object (for example order document) is posted and the message is processed automatically.
Prerequisites
Your application must call the corresponding functions of the Message Control. There are also applications which transfer the found message directly for processing without offering any manual processing options. The message is therefore also called the „message default", although you cannot change the message in any way.
Features
The created message default contains the recipient of the message, the transmission medium and the processing time for the subsequent (automatic) processing. You can change these parameters in the transaction application.
You can display a determination analysis showing whether messages were found and if so where it was in the procedure. You can display a message in detail which will show which application data determined the condition table key.
After document posting the processed message default is stored as a
Message Control record in table NAST.
In SD a sales order is placed in sales organization 0001 with sold-to party CUSTOMER1. The system suggests a purchase order acknowledgment (output type BA00) that should be sent by EDI.
The determination analysis shows that the key of the condition table consisting of the fields "sales organization" and "customer" is determined with the application fields "sales organization" (0001) and "sold-to party" (CUSTOMER1). The condition table is read with this key, a condition record is found and proposed in SD as an EDI purchase order acknowledgment.