Aircraft have two levels of emergency - "Mayday" and "Pan Pan", but I've been told that as far as ATC is concerned, they treat them both exactly the same.

The question is not, "can we have multiple levels or different codes in a PLB?", but "is it useful to have multiple codes?" I don't believe the S&R authorities would want to have this, for various reasons.

1. Ambulances are manned and dispatched by trained crews; they use the Code system on a regular basis. Moreover, they get enough calls in the course of a day that it's important for them to prioritise.

2. Most people who will be using PLBs won't be experienced in their use, they'll be in what they consider to be dire straits. Asking them to make a decision as to whether it's a class I, class II, or class III emergency is asking too much, IMO.

3. for the most part, "mayday" type emergencies don't happen simultaneously so there's little need for S&R to prioritise them. In the cases where S&R receive multiple mayday calls simultaneously, it'll be cases like the 1998 Sydney to Hobart Yacht Race disaster. In that instance, one of the biggest problems rescuers faced was that sailors who were rescued off their boats didn't take the time to turn off their emergency beacons; so rescuers the next day were still investigating beacons from derelict ships. If they can't even keep their heads enough to turn off their beacons when being rescued, the chances of them being able to make a rational decision as to what degree of emergency they should announce will be ridiculously low, IMO.

4. I'm pretty sure the rescuers will ignore the level of signal from the ground, just as ATC ignores the distinction between a Pan Pan and a Mayday call. (I'm pretty sure ambulance dispatchers assign a priority based on the caller's description of the incident, and not on how quickly the caller wants the ambulance to be there.)