The .NET Scripting Backend is using Microsofts .NET to power scripting. Unlike other Unity platforms, Windows Store is not using .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.