As we mentioned previously, UWP supports a wide range of devices. Each of these devices executes its own implementation of .NET Standard and the UWP app model.

Nevertheless, the surface area of this complete API layer might not always apply to the target platform. The UWP app model contains certain APIs that are specific to only a subset of these devices. These types of API modules are in fact left as placeholder methods in the core UWP SDK, while the actual implementation is included in extension modules that can be referenced in your UWP applications:

Without adding the specific SDK, the developers are confined to only universal APIs. Without adding the extension modules, it is highly likely that certain platform-specific methods would throw NotImplementedException or similar, since the actual implementation of these methods only exists in the platform extensions...