This is the best explaination i have found to date.
[The BR9ES, BR9ECS, BR9EYA and BR9EIX are all basically the same plug.
The B stands for 14 mm thread diameter
The R means resistor type
The 9 (or number) is the heat range The larger the number the colder the heat range. The spark plug does not add heat to the engine other than igniting the fuel/air mixture. The heat range refers to the plugs ability to remove heat from the combustion chamber. The colder plugs remove more heat and therefore are less likely to detonate.
The E stands for 19 mm (3/4") reach
The S stands for standard electrode
The C stands for low angle ground electrode
The Y stands for v groove center electrode
The A stand for special design
The I stands for Iridium electrode
The X stands for booster gap
Now to explain the differences. The ECS is the same as the ES except for the chamfer and the ground electrode is welded more securely to the flat section. This was designed by NGK to eliminate electrodes from burning off in combustion chamber. I guess they had issues during initial testing of the SDI engines. The gaps can not be changed on the ECS because you would cause the electrode weld to bend and fail. The chamfer was also supposed to reduce hot spots near the center of combustion chamber therefore reducing detonation. Evidently the electrodes don't burn off under normal conditions, otherwise we would hear about it with all the different plugs being tested.
The ES and EYA are the same except for the electrode on the EYA is "V" grooved. The idea of the V grooved electode is to force the spark to the outer edges of the ground electrode placing it closer to the fuel/air mixture giving more complete combustion.
The EYA and EIX are the same except the EIX has an Iridium electrode. Iridium is a very dense material and requires less voltage to spark. It burns very efficiently and sparks well at leaner fuel mixtures. Most Asian motorcycles perform extremely well running Iridium plugs. Very expensive and high performance.]
Take it for what its worth...