The MSTJCLxx parmlib member contains the master scheduler job control language (JCL) that controls system initialization and processing. You can place the master scheduler JCL in MSTJCLxx as an alternative to keeping it in the MSTJCLxx module in SYS1.LINKLIB.
The advantage to placing the master JCL in the MSTJCLxx parmlib member is that it is easier to make changes to the master JCL. If you specify the master JCL in the MSTJCLxx module in linklib, you must reassemble the module and link-edit it into the system each time a change is made. Specifying the master JCL in parmlib eliminates the need to reassemble and link-edit the module.
If the system finds a MSTJCLxx parmlib member, it uses the JCL in that member to start the master scheduler address space. If the system does not find a MSTJCLxx parmlib member, it uses the JCL in the MSTJCLxx module in linklib.
For more information about how to change the master JCL or specify alternate versions of the master JCL data set, see Understanding the master scheduler job control language.