Don't know that much but on Wikipedia two door sedan is defined like this
The Society of Automotive Engineers defines such a vehicle as any two-door model with rear accommodation greater than or equal to 33 cubic feet (930 L) in volume (a calculation made by adding the legroom, shoulder room, and headroom).[citation needed] By this standard, the Chevrolet Monte Carlo, Ferrari 612 Scaglietti, and Mercedes-Benz CL-Class coupés are all two-door sedans. Only a few sources, however (including the magazine Car and Driver), use the two-door sedan label in this manner.
In the popular vernacular, a two-door sedan is defined by appearance and not by volume; vehicles with a B-pillar between the front and rear windows are generally called two-door sedans, while hardtops (without the pillar, and often incorporating a sloping backlight) are called coupés.
The Mazda RX-8 meets the volume requirement to be called a sedan, but it has vestigial rear-hinged rear doors, so some call it 2+2-door sedan. Another term for a coupé endowed with rear-hinged doors is a "quad coupé." However, this may simply be vernacular, based on a possible copyright by General Motors, for its Saturn Ion Quad-Coupe.
http://en.wikipedia.org/wiki/Sedan_(car)
