Event tracing is automatically initiated on every IPL of VM370 and the MONITOR command allows changing the
options and events involved with this tracing or terminating it all together. The trace is primarily
used by system programmers to track the cause of system failures. The options in effect when it is
started by a VM370 IPL are defined at system generation time. The MONITOR command is also used to
collect performance statistics.
Reference the detailed IBM manaul page here.