Resources in a message set are created as files, and are displayed under the message set folder in the Application Development view.
There is always one, and only one, messageSet.mset file in a message set. This file contains message model properties that are common to all the content of the message set. It is also where you define the physical formats that you want for this message set. These can be Custom Wire Format (CWF), Tagged Delimited String Format (TDS), and XML Wire Format (XML).
The file is created for you when a new message set is created, and you manipulate its content with the Message Set Editor.
You can have many message definition files in a message set. Each file contains the logical model and the associated physical model, in XML Schema form, for a group of related messages.
These files are used by the SOAP domain. You can have many WSDL files in a message set.
These files are optional. You can have many message category files in a message set. A message category provides another way of grouping your messages, perhaps for documentation purposes.