The .NET scripting backend uses Microsoft’s .NET to power scripting. Unlike other Unity platforms, Universal Windows Platform does not use the .NET 3.5 Class Libraries profile, but rather a subset of .NET 4.5. This is due to Windows Store certification requirements. This means that the platform has some incompatibilities compared to other platforms, such as different plugin model and many missing types that are present in .NET 3.5 profile that Unity normally uses.
Note: The .NET scripting backend is deprecated and will be removed in a future release of Unity. We recommend switching to the IL2CPP scripting backend.