What is message type?

The message type defines the semantic context of an IDoc. The message type tells
the processing routines, how the message has to be interpreted.
The same IDoc data can be sent with different message types. E.g. The same IDoc
structure which is used for a purchase order can also be used for transmitting a sales
order. Imagine the situation that you receive a sales order from your clients and in
addition you receive copies of sales orders sent by an subsidiary of your company.