Notes:
Assuming the QE14 and K21AP fares’ rules are met, there is another valid way to pay for the same flights, using a BOS-DFW fare to pay for the first flight of the return and a DFW-SFO fare to pay for the second flight. In this case the total is only $1800.
In general, a traveler may choose whatever fare combination for a ticket they wish so long as all fare rules are satisfied: usually they’ll want to choose the cheapest combination, but (for example) they may choose more expensive fares that permit refunds or first-class travel if they so desire.