The CA OPS/MVS Event Management and Automation Parameter Reference Guide explains that the SSIMSG Parameter determines whether CA OPS/MVS processes messages before or after JES2 processes them.
This parameter has 2 possible values, NO and YES.
When SSIMSG Parameter is set to YES CA OPS/MVS processes messages before JES2 does so CA OPS/MVS MSG.JOBNM environmental variable is not populated .
What could be the reason to have information in the MSG.JOBNM environmental variable when SSIMSG Parameter is set to YES ?
With OPS/MVS parameter SSIMSG set to YES, there will be some messages that do not have msg.jobnm populated. This depends on if the asid/job has requested and received a Job number from JES when the message was issued.
This depends whether the GETJOBID subsystem call was done before the message was issued or not.
The definition of SSIMSG Parameter can be found at the following link: