Nice! I just had a similar question engine rebuild and more...
The NGK DPR7
EA-9 are for cold climates
EA-9 are the stock spec
EA-9 are for "performance" bikes
I'm also curious what these temp ratings mean. I guess I'm more interested in the fundamental "why", that is to say what is the difference between these plugs that makes them suited for these different environments? Sorry if I'm hijacking your thread, just tell me to shut up.
But to answer your question, the DPR8EA-9 is the spec plug. But as rosco said, it depends on climate and riding environment. How? IDK, but that's what I'm after.
the 7,8,and 9 are the temperature ratings. On the ngk the plugs are on a scale of 2 to 13, 2 being the hottest and 13 the coldest. The insulator of a spark plug is a very poor conductor of heat, so it uses the metal of the spark plug to dissipate the heat. On the hotter spark plugs the there is less of the insulator tip touching the metal of the spark plug. With less surface contact there is less transference of heat thus creating a "hotter" spark plug. A colder spark plug is just the opposite there is more surface contact in between the insulator tip and the metal of the spark plug allowing more heat to escape.
Now on are engines the the different heat ranges called for in different climates and driving conditions are easy to figure out. If you live in an average climate, and do shorter stop and go city type driving you want to use a the DPR8
EA-9. in a colder climate and sometime an older engine you want to use the DPR7
EA-9 this helps keep everything running around optimal temperature and performance. On older engines, running rich or leaky oil rings, the hotter temperature helps to keep the spark plug clean instead of fouling it out, not the proper fix obviously but is one solution. In a hotter climate or on a motorcycle driving at higher RPMs for longer period of times, high performance or highway driving, you want to use the DPR9
EA-9, what this will do is help from heating the cylinder up to much and causing engine knocking or pinging, preignition where the fuel air mixture ignites before the spark plug fires witch can cause the temperature to raise significantly in the cylinder causing burnt valves or even melt parts.
hope this helps more than confuses you.