Engine Spark Plug Question

Spark Plug Question

hey ive got some questions about the plugs in a 98-02 rolla.
first off what is the heat range that they fall under stock, and whats the point in changing the heat range, i know a lot of people say to go colder if using forced induction, but will going colder hurt if your not running forced induction? If anyone knows the actual tech info then let me know im curious.

and i know greddy makes some plugs for the car in 2 different models ISO and JIS does anyone know what the difference is? just wondering cuz i'd like to use the greddy plugs since i have a lot of other greddy accessories, catch can, oil cap, radiator cap, and greddy perf oil.
thanks