SNELL 2K is what matters. Fancy materials are not necesarily any better than proven ones. The SNELL group doesn't care what a helmet is made of as long as it protects your head!
The one exception to that is M (Motercycle) vs. SA (Cars). SAs are required to be more fire resistant because of the enclosed space vs. a motorcycle.
Most amateur events allow M rated helmets for autocross and HPDE type events, but some require SA.
Your milage may very.
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