I remember booking an intra-European itinerary in booking class J: I traveled in business class, indeed. However, I recall the fare being called "business saver" (or super-saver), and the description saying that J was actually an economy fare bucket eligible for an instant upgrade. This was two years ago, so please allow my memory some latitude: what I mean to say is that the J fare bucket is probably a business upgrade based on an economy fare, meaning a business class seat would be granted based on availability at the time of booking. Hope this helps solve the mistery.