In current usage J and C seem to have become equivalent. Historically C was the standard full fare flexible business class, and J was 10% higher fare for a "premium" product when various airlines introduced first-generation improved business seats (pre-beds). Now some airlines use C, some use J. Cannot remember any airline that uses both J and C; and they seem to be completely equivalent on most airlines. D is usually discounted and refundable/changeable (with a fee) while I is a greater discount but non-refundable and often allowing no changes (not even date!).