Personal Locator Beacons (PLBs) can be carried during outdoor activities to use in case of an emergency. But are there any guidelines on what exactly constitutes an "emergency"? For example, what about the following situations:
You slip and fall on a trail several hours away from the trailhead, breaking several bones and putting you in intense pain. Should you hobble back to the trailhead to call for help or activate the PLB?
You are bitten by a rattlesnake, experience symptoms of envenomation, and are several hours away from your car. Do you attempt to hike back and call for help or activate the PLB?
While skiing in the morning in an isolated area of a ski resort, you run into a tree and break bones. No one responds to calls for help. Do you activate the PLB or wait until nightfall to see if anyone finds you?
Those are kind of contrived examples, but basically when is it appropriate to use a PLB? If the answers are different for different countries, I'm wondering specifically about use in the US.