JDM stands for   ”Japanese Domestic Market”  and there is a reason why such cars as a Honda, Nissan, Toyota, Mazda, Subaru  and Mitsubishi are popular among the JDM culture,   and that’s because the octane level in the Japanese petrol is higer than most of the other octane levels worldwide.   Generally speaking this means that the engines actually have to be  de-tuned  to be used in other contries with lower octane levels.

if you  get your jdm car imported strait from japan, then you will have plenty of opportunitys to  Tune and modify your JDM car.   and that is where the JDM culture was born.

This explanation was taken by   GasKings   youtube