Thanks! If the arrival (Y) is a specific location, the return flight is guaranteed to be Y->X.

However, if Y is a country, region, or continent, the return flight could be from any airport within that area, prioritizing the cheapest option. For example, if you search for a round trip: "Berlin to Asia for a week in Aug" the arrival could be Bangkok, but the departure for your return flight could be from Singapore.

Now that you mention it, I wonder if it would be better to enforce return_from_same_city or allow the user to toggle this option. What do you think?