Lots of airlines zero out discount inventory on specific days of the week or on flights by time of day. For example QF has some flights that have no A or D inventory on Fridays (but wide open for other dates), and AA has flights with no D during morning/afternoon rush, but open during mid-day. The idea is the same as for discounted Y fare codes: the cheaper fares are funneled to the less desirable flights, leaving premium seats empty for sale at a higher fare. The POS wrinkle allows distinguishing the same fare code into a lower priced fare code, and making seats available only for higher fares. Whether this practice is well implmented is a different issue. I suppose the ultimate implementation would be to assign a dollar value to the sector, and bid for the seats based on the dollar cost of the ticket. At that point Y tickets with upgrades will beat out some of the D fares!