Originally Posted by Starman

This surprises me. If I understand your explanation, this implies that one could use cheap S/T fares that were intended to be roundtrip sale only for one-way trips, by (assuming they are combinable with Y) buying S/T outbound and Y for return, then refunding the unused Y, effectively having bought a one-way S/T. I don't think you can get away with this. The rule you've quoted discusses the size of the penalty/change fee - but are you also suggesting that refundability and advance-purchase remain unrestricted if one uses S outbound as originally ticketed, and then wants to change/refund the Y return?