Hot or cold refers to the plugs ability to dissipate heat. A cold plug is designed to transfer more heat than a hot plug and under the same set of conditions the cold plug would operate at a lower temperature. Glow plugs that operate in high nitro, high compression envoironments need to transfer a lot of heat so that they do not contribute to detonation or simply fail prematurely. That is why high performance engines ( such as racing) use cold plugs. On the other hand low nitro, low compression engine glow plugs need to retain heat in order to facilitate a smooth running engine from idle, transition and hgher speeds.