GENERALLY, a hot plug is used with low percentage or no nitro fuel and a cold plug is used with high percentage nitro fuel. There are a multitude of variables in between, such as engine compression, barometric pressure/air temperature, type of exhaust system used, prop used, etc., but that is the basic idea. An engine using low nitro content fuel will generally have a lower internal combustion temperature and often needs a "hot" plug to run/idle properly. If you used the same hot plug with high nitro content fuel, you would run the risk of overheating the engine (nitro basically makes the combustion hotter).

Not unlike hot and cold plugs used for automobiles.

Does that answer your basic question? If you need more specifics, I'd suggest seeking assistance from someone with a more "scientific" background who is knowledgeable/has expertise in this area.

Hope this helps!