SA = rated for auto, meaning Nomex fireproof interior, also suitable for karting
K = rated for karting, non-fireproof interior, NOT suitable for auto
For Arai, the shells are identical between their car and karting helmets with the only difference being in ventilation and interior material. Most companies do the same thing.
Also, depending on which SA model you buy, there could be eyeport difference or other subtle things like that which cater more to the type of driving you’re doing. A helmet that is geared toward karting or open cockpit driving will usually feature a smaller eyeport. A helmet that is geared more toward closed-cockpit driving like GTs or touring cars will usually have a larger eye port and more ventilation. Smaller eyeport = more protection from flying debris. A larger eyeport obviously allows more range of sight for the driver.
The other thing I’d like to note is the idea of using motorcycle helmets for karting. I think it’s legal in most orgs. Some people have said they prefer motorcycle helmets over karting helmets or vice versa. It comes down to preference for the most part, but motorcycle helmets typically have a little different weight balance on your head and a different shaped eyeport compared to a karting helmet, because the driver/rider posture is very different in each one. Typically you’re looking up and forward in a motorcycle helmet because you’re leaning forward. In a kart you’re sitting back more and look more directly ahead.
Just some background info from someone who has disassembled hundreds of helmets and probably has a fair bit of behind the scenes info because that!