The idea is that the app is supposed to handle the case when it is trying to use a functionality present only in a newer API contract version. The app needs to (and can) now check whether the target device contains that functionality or not.
@bdhc73a:You can find a lot of documentation about all Windows 10 API (not only XAML) here:
API contracts and adaptive apps are relevant only for Windows 10 and above
@Brigadir: Using C++/CX makes it much easier to deal with any Windows Runtime API and the concept of API contracts. It is possible to do all of this using the WRL library also but the experience for C++/CX is slightly better integrated with the Visual Studio IDE.