Plug electrode heat build up transfers through the porcelain to the plug main body. The farther it is from the main body, the less heat transfer there will be and the hotter the electrode will run. What I was always taught.
I have not been sure what part of plug is used for the heat rating. Found this link this evening:
https://www.ngksparkplugs.com/about-ngk/tech-talk/spark-plug-basicsThe description that NGK provides indicates to me that the heat rating does reflect the tip/electrode end of the plug. That should mean that a projected and non-projected tip plug of the same heat rating would have approximately the same temperature at the electrode. That temperature point is placed in a different location in the combustion chamber though.
