it has the mapping of the ignition,, different maps give a different "feel" to the bike, or changes the way the bike builds power. the 05 and newer bikes have a cdi with 2 different maps that can be switced via a bar mounted switch. Rainy mode-smoother and sunny mode-crisper..
The calmest cdi I believe is the 03 ec 250 as that caused the most issues,, I swapped over to one from an 01 300xc and it really woke up the bottom end.