C grain is just like the rest of balsa, it comes in light as well as heavy densities. "Contest grade" does NOT refer to C grain. Contest grade is a term from the old free flight days where the guys wanted the lightest and finest wood. So contest grade means it's 4 to 6 lb/cuft density and generally of higher quality grain wise and has little built in warping in the sheet. C grain is merely cut "cross grain". Nothing more or less.