540 CMR, § 7.03
Type A. The Type "A" school bus is a conversion or body constructed on a van-type compact truck or a front-section vehicle, with a gross vehicle weight rating of 14,500 lbs. or less, designed for carrying more than ten persons.
Type B. The Type "B" school bus is constructed utilizing a stripped chassis with a gross vehicle weight rating greater than 12,500 lbs. The entrance door is behind the front wheels.
Type C. The Type "C" school bus is a body installed on a flat back cowl chassis with a gross vehicle weight rating of more than 12,500 lbs., designed for carrying more than ten persons. In a Type C school bus, all of the engine is in front of the windshield and the entrance door is behind the front wheels. (May have a left side driver door).
Type D. The Type "D" school bus is a body installed on a chassis, with the engine mounted in the front, midship, or rear, with a gross vehicle weight rating of more than 12,500 lbs., designed for carrying more than ten people. In a Type D school bus, the engine may be behind the windshield and beside the driver's seat or at the rear of the bus behind the rear wheels, or midship between the front and rear axles. The entrance door is ahead of the front wheels.
540 CMR, § 7.03