Since I am pretty new still to this forum, I might not know all the details about the codes. But I have flown recently several times in Europe on a Z fare. This shows up to me as Business Discounted. It is non refundable, non changeable. I dont think I have seen J on any flights I have been on this far.
Also, my tickets has been booked by a travel agency. Checking myself, I have not seen any Z fares either. Dont know if this helps in any way