To go one further; there are two functions of the heat range of the plug. How much heat the body retains, and how “hot” the wire is. Some manufacturers make plugs such that there are hot body and cold body plugs, and also hot, medium, and cold elements. You could get a cold body hot element plug, hot body medium element (my preference), etc. Its just more way to fine tune the engine.
Generally, the glow plug heat range is chosen based on the methanol content. The higher the methanol (lower nitro) content, the hotter the plug needed. Methanol itself burns pretty slow, so more heat is needed.