Easy stupid TIR question

Sorry, I've tried to find the answer to this question and even tried AI, but can't get a straight answer.

What is the angle (stated degrees) of a TIR actually measuring? My guess would be that it's either (A) the angle between straight forwards and the side of the hotspot cone, or it's (B) the angle from one side of the hotspot cone to the other side of the hotspot cone. I'm leaning towards B but wanted to be sure. Or it might be a different measured angle altogether.