Where I am in Europe diesels are king, and usually people import 2nd or 3rd hand cars from Germany. I'm from the UK originally, but it's kind of funny that I see a couple of magnitudes more BMWs here, which is much poorer country.
Once your engine has done two or three hundred thousand kilometers the original tuning doesn't really work that great, so you probably want to remap it. This is not only to increase performance, it is also done to decrease fuel consumption and burn cleaner (the technical inspection here is very strict about CO) or makes it start easier when cold.
This is done using software that lets you change how much fuel is injected across different RPMs and throttle levels. If you car has an Eco mode, this is how that works. The thing that there are only really 5 manufacturers of diesel engines for cars in Europe: Volkswagen (VW, Seat, Skoda, Audi, Porsche), PSA (Pegeuot, Citreon, Fiat, Opel, Ford), Mercedes (Mercedes, Nissan, Renault), BMW and Volvo; and the way they work under the hood is all pretty similar.
The fuel injection system and control equipment is not developed in house, but made by someone like Bosch, so if you have two engines made by differnet manufacturers but using an ECU from the same supplier, as you can understand there will be similarities.
To be able to tune engines properly you not only need to know how to edit the map, but also all the perculiarities of that particular engine. A lot of it is trial and error and reading Russian forums (where most the scene is developed).
As you can probably guess it's also used to work around emissions control devices, which are usually no longer functional when you're engine has done a few hundred thousand kilometers, and at this point can actually increase fuel consumption and can damage the engine. For some reason manufacturers do not sell replacement parts, so if you want to keep the engine running your only option is to disable it in software and have it physically removed. Because of all this it's common to see 20 year old cars still driving that have done over half a million kilometers.