What is a passenger disturbance level? I was listening to RealATC for a United Flight UAL1074 which declared an emergency needing to return to Washington Reagan National Airport. During the ATC recording, the flight declares an emergency due to a passenger disruption.

ATC asks for something called the Passenger Disturbance Level. The Copilot replies with a Disturbance Level of 2. As I had never heard this asked for before, I was curious as to what the levels are for and what they each mean?
The FAA has categorized disturbances into four levels:


  Level 1: Disruptive behavior - suspicious or threatening
    Level 2: Physically abusive behavior
    Level 3: Life-threatening behavior
    Level 4: Attempted or actual breach of the flight deck 


These are also ICAO standard classifications from the restricted Doc 9811, according to IATA. I'm not sure who came up with them first. That IATA document also provides sample things that might be considered to be each level.