Not an expert by any means, but I think it's generally used for "fare buckets" in which each might have 9 tickets available at certain prices. A flight could have J9, I9, C9, Z9 and once all Z fares are sold (at the same price) they sell the next fare at a higher price. That's why C and I tickets have same earnings but their difference would be price - based on the other being sold out.