PMON also uses parallel executions slaves to do transaction rollback. You can trace it with event 10218. This event causes the undo block address of applied undo to be written to the trace file of the process applying it. Set the event system wide and kill an active session. You'll see the trace of undo applied in the trace file of the parallel execution slave process used. So far as I am aware, this is not documented anywhere other than on the Ixora web site.

Regarding what you said about PMON parallel rollback, you are refering to the parameter fast_start_parallel_rollback aren't you? Because this is for parallel rollback, but it is coordinated by SMON.