The tip of the spark plug needs to be kept at a reasonable temperature - about 650C if I remember right. Basically but not covering everything : .....
If it runs too cool it will be prone to fouling building up on it, which may lead to the electrical energy travelling through the fouling or across it's surface as the path of least resistance to the metal shell of the spark plug rather than jumping the gap, hence no sparks.
If it runs too hot this will lead to excessive electrode wear and a fraction hotter it will lead to pre-ignition where the fuel-air mixture ignites under compression on this hot spot at the wrong time instead of when the spark happens later in the cycle, an extreme case of this can burn a hole in the top of the piston
The difference between plugs in a temperature range is the length of the thermal path from the tip to the threaded body shell, the longer the insulator path the hotter the tip will run under the same conditions.
All sorts of things effect this, on the heat creation side, fuel mixture, ignition timing, compression ratio, and engine load being main ones. On the other side the rate that heat is dispersed away from the plug through the cylinder head.
So ultimately you want a plug that stays in the self cleaning sweet spot without overheating through the load range for that engine,
Yam will have spent some time assessing that, I hope
Oh, PS, look out some plug manufacturers run their numbers the other way ie higher = hotter
