HR's are the bottom of the barrel, right above the Basics. They're single height adjustable with no damping adjustment. HA's are a step up with the same 6k/5k spring rate, but with 16-way damping adjustment. HA's are still only have single height adjustment. HE's are double height adjustable, 16-way damped, and have a higher spring rate (8k/6k) more suitable for drift and tracks. All Flexs should be EDFC compatible. They are double height adjustable, 16-way damped, and have street friendly 5k/4k spring rates. There is a new version of the Flex available for S13 that has 7k/7k springs, which I would think would be oversteer monsters.

*edit* changed the title of the thread to "FS"