I noticed that the MQ 9.2 documentation for z/OS refers to different message classes. This is nothing very exciting. From the JCL it says
This sample shows 8 other page sets, three for each major class of message, and one for each other class:
- 00 – object definitions
- 01 – system-related messages
- 02-04 – important long-lived messages
- 05-07 – short-lived messages
- 08 – miscellaneous messages
Basically it means provide multiple page sets for the major classes of messages. This gives more capacity and can spread the I/O work load.