I would hazard a guess of an engine mis-behaving but not a failure or worse, and not a simple case of when a parameter reaches a limit and is covered by QRH. This could include oil issues, VIB's of uncertain nature, mysterious noises but not a certain surge/stall. I think they've left it open for pilot discretion.
It is an educational scenario in the sim to give a failure that results in ending up with an engine running at idle or much reduced thrust and watch what the crew decides to do about it. This is not a simple basic QRH scenario and requires good decision making and crew coordination.