In some/many parts of the world round trip (return) tickets are cheaper than 2 one ways. Some airlines have interline agreements with other airlines so that when a route is split between 2 airline partners, the revenue is also split at an agreed pro-rate. Example Moscow SV HEL SK STO. In this case the passenger pays a one-way fare from Moscow to Stockholm and the revenue is split between the 2 airlines - not always 50/50 - depends on the route. For example it would be unfair to Aeroflot if Virgin took 50% of the MAN/MOW fare on MAN/LON/MOW. An interline agreement is therefore negotiated between the 2 (or more airlines). However, some airlines will only accept pro-rate for a round trip ticket if they carry the passenger for 50% of the journey and get half the revenue. I seem to remember that QF fares from Australia to Europe could be interlined to other carriers at stopover points ie SYD QF HKG BA LON QF SYD. However, LH, for example would only accept half the return fare as the pro-rate so SYD QF LON LH FRA LH SYD would be ok (half a return fare goes to LH) but SYD LON BA FRA LH SYD would not be permitted as LH wouldnt get half the SYD/LON fare. This may be an over simplification but I'm not sure how much IATA fares knowledge you already have. Depending on the fare calculation the Russian airline uses, the rules of the fare may not be in Russian only in English so it's quite possible there is no 1-word Russian language equivalent. U datcha!