Also go by whichever org (NHRA-IHRA) has the tightest rules and not the more lax requirments---that way one is never caught off guard.
i got my car certified this weekend and even while at the track no one could give me a straight answer on which suit I need, I know the safer the better but its hard to want to spend $1200+ on a suit when I could spend less than half that and be out racing. from what I read in the rule book it says that in an altered with a supercharged engine a 15 suit is required but it doesn't say anything about a naturally aspirated motor. at this point the suit is really the only thing holding me up from racing.