For example, BOS=PVG (route Via EWR/SFO/ORD), normally 1300 in K fare.
BOS=CTU (route via NRT and connect to NH or route via PEK ad connect to Air China), the same fare code(K) as BOS=EWR=PVG part,but PVG=CTU is full Y. So the total price draws up to BOS=EWR=PVG=CTU = 1300USD+1200USD=2500USD. However there is alway discounted PVG=CTU which has K or other Fare codes on Air China(to comply with United's K) Why United offer Partner's Y instead of all the same fare code i.e K on united and air china or K on united and Y on Airchina .