The RBD should be the reservation booking designator. I believe what it means is if you book on "economy smart" and there is a fare difference in "economy smart", then it will be waived. However, if you book on "economy promo" then it had to be rebook into "economy smart", then the waiver of fee difference will not be applied. The only unsure if you are rebook from V class into another group of K/M/L/V class that is other than V class, will you get the waiver?This thing getting complicated these days as there are too many booking class type in economy class...