These cars are developed especially for European market.
Here BMW doesn't sell a lot of cars with large petrol engines due to the prices of the fuel.
So 35d/40d models were near v8 diesels from Audi and Merc, but the level of power was lower for the BMW engine: 313hp vs 350hp for Audi.
So BMW came with 50d nameing offering better performances and fuel consumption, also trying to fill the gap to real M models which are not the best sellers in Europe vs Asia or NA.
The best way to buy these cars is to debadge them when ordering and they will look more interesting for non BMW connaiseurs