From the horse's mouth (or rather, email address):

We offer 5 types of booking class on our business class service as follows:
(1) C - is full fare business class booking class
(2) J - is business class fare with restriction/s booking class
(3) D - is business class round-the-world booking class, and selected Interline Sales
(4) I - is business class redemption booking class
(5) Z - is business class on A345 non-stop flight between Singapore-Los Angeles, and vv.