Booking codes are basically a price code for fares, but it sometimes gets confused with using letters to generically describe what class of service or cabin you'll be seated in. IE- F J Y describe US first, business and economy class cabins (F,C,Y ex-US). But the booking codes for your fare are much more numerous. You could have a biz class ticket but it could be booked for instance on AA in J or D or I or U.