The SCCA rulebook was very clear (40 years ago):
Sports cars do not have back seats of any kind.
Sedans have "normal" back seats (i.e. comparable to the front seats).
GT cars have back seats for "occasional use" (i.e., flip down, padded cardboard, always smaller and less comfortable than fronts). Example: Jag XKE is a sports car, Jaguar XKE 2+2 is a GT car.
What they did to classify a Camaro (defined by DOT as 4-5 passenger) would be largely politics: if enough big sponsors vapor-lock, it's a GT car. If not (or the winning GT sponsors have even more money) it's a sedan.