For 21 class racing engines of T~type inner head. Use when atmospheric temperature
If you have one eye on the weather chart and want to maximise your glow plug choice, this plug is for when the atmospheric pressure is high.
Glow plugs are considered to be consumables - But by using a low voltage to start ( under 1.5 volts) and not burning the plug dry, you prolong the life of your glow plug. It is also worth washing out a new engine after running in to prevent small particles shorting the plug and destroying it. Blowing plugs on new engines is less common now a days, but is still something that can happen.