The transmission priority (TMSPTY) parameter is on the class-of-service (COS) description. When you create a class-of-service description, you can define one of three transmission priorities for each class of service. You can specify, by the transmission priority (TMSPTY) parameter, that the transmission priority for any class of service is high, medium, or low.
The session activation request carries the transmission priority you specify when a session is established. This allows each logical unit on the session and each routing entry along the session path to store the same transmission priority. You can ensure better response time for the applications that require it by assigning an appropriate mode (which includes a class of service) at session activation time.