DL is fare difference (generally doing a reasonable job of capturing booking-time availability: at least there isn't the volume of complaints about the fare difference being calculated between an advance-purchase Z fare and a walk-up M fare... that nearly all DL premium fares are dual-inventory may simplify this: a Z fare that requires T inventory being booked means it's likely that you can use the T fares to calculate the difference) as a refund plus a $200 credit.