Since a subsystem name other than the default OPSS was used, the S in the message was replaced with the subsystem identifier. In the example above it was with X to show that it occurred on OPSX as evidenced in the message, OPX8500I Global variable backup for subsystem OPSX has started.
For more information on the messaging formats used by OPS/MVS Performance Manager, review the section of documentation titled "Understanding OPS/MVS Messages".