FWIW, this is the way it was explained to me many years ago. It's a 'convertible' if the top can go Down and the side windows can go Up, making it capable of being converted from a 'closed car' to an 'open car'. If it doesn't have side windows, it's considered a 'roadster'. This was confusing to me as a young kid because the early Corvettes with tops and windows were called roadsters. But as HRP mentioned, it's up to the show organizers and judges, it's their definition that is going to count, not ours.