I think a while ago we determined that CB stood for "Central Bobbin", at least for the Singers.
http://www.quiltingboard.com/vintage...e-t216990.html
I honestly don't know why Bernina made that distinction. I can't see a difference between those 15 CB bobbins and a regular 15 bobbin. I've ordered the actual ones for a customer at one point and there was no discernible difference that I could come up with (I didn't pull out calipers) besides maybe the number of holes on each side of the bobbin which should make little to no difference. I think it's mostly a "if we make it sound different, they'll buy our bobbins instead of the regular class 15 bobbins" just like Pfaff, Kenmore, Janome, etc.