Message category files have the suffix .category. 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 or convenience purposes, or for assisting in the generation of Web Services Description Language (WSDL) files.
A message set category file is created using the New Message Category File wizard.
When you have created your message category file, you must specify one key property.
You can then add messages to the message category file. If the message category is to participate in WSDL generation, you should assign appropriate values to the Role Name and Role Type properties of each member message.