not an expert by any means, but I believe one of em is an open class, MX3 I think it is. It probably used to mean something much different before the 450 4 stroke revolution, now it seems most are on the 450 in either mx1 or mx3.
I can see where the confussion comes from. The 450 4-stroke has taken over the 250 2-stroke class. Since no one "really" makes a 500cc 2-stroke, the 450 4-stroke is also the open class bike. Now wouldn't it be cool to see a Pro rider show up with a CR or KX AF :cool: