Actually the Conference Rule Book does not specifically define what a closed car is. There are specific rules that define what a Sports Racer is and excludes Production Based cars no matter how extensively modified. So if a Sports Racer with a closed top has to compete under Closed Wheel car rules, that would imply that closed top Sports Racers can compete outside of groups 3 and 6? And if they could, then open Sports Racers should as well, since Convertible Closed Wheel cars are allowed to use Open Sports Racer front hoop/rear hoop roll cage construction.
Just Kidding.
In the 2015 ICSCC Rule Book these FIA Articles apply to sports racers:
Application of FIA Appendix C to ICSCC Sports Racing Cars
Page 112
Art. 207, altered as follows:
Closed cars shall be equipped with at least one rigid door affording the driver easy access to the
driving position.
Page 113
Art.211:
Closed cars: Bodies of closed cars must correspond at least to all the conditions indicated
above for open cars and must be established in such a way that they insure adequate and safe
visibility for the driver.
During races either by means of open windows or by a special apparatus, a sufficient draft must
exist to prevent gases from accumulating inside the car.
There is no specific mention of windows down for safety. Only the need for an opening door, safe visibility, and an opening for ventilation. Though they are FIA rules, and might seemingly be a conflict as Dan mentions per SCCA conflicts being over-ridden by Conference Production Car Rules, Articles 207 and 211 "altered as follows" are ICSCC sanctioned.
So Closed Top Sports Racers have a Conference Approved Provision to run with closed windows, with the option of venting in the windows if the driver so chooses.
Andre