Originally Posted by jimrpa

Delta's "discounted" business fares are D and I. D is slightly discounted, I is significantly discounted, with significant restrictions. Both fares are eligible for all the regular COS bonuses, etc. Other than reward fares (O), Delta does not have any published purchasable business class fares that do not qualify for the business class bonuses.

Many years ago, Delta was experimenting with a system whereby the lowest COACH fares (L, U, and T) would only qualify for 1/2 the MQMs (and mileage) of a flight. Hence, the mnemonic ".5LUT" fares came into being.