To build a Windows Phone 8.1 application, select “Phone 8.1” SDK under Windows Store platform.
Windows Store.
UNITY_METRO, UNITY_WP_8_1, UNITY_WINRT, UNITY_WINRT_8_1. Additionally, NETFX_CORE is defined when compiling scripts with Microsoft .NET compiler.
Under publishing settings on Windows Store player settings, there’s a drop down menu called “Compilation overrides”. There are 3 settings:
1. None. All C# scripts will get compiled with Mono C# compiler;
2. Use Net Core Partially. Scripts that are in folders “Assets/Plugins”, “Assets/Standard Assets” and “Assets/Pro Standard Assets” will get compiled with Mono C# compiler, while the rest will be compiled with Microsoft C# compiler;
3. Use Net Core. All scripts will get compiled with Microsoft C# compiler.
Both compilers have their ups and downs. Compiling scripts with the Mono C# compiler will allow them to be referenced by JavaScript scripts, which, for example, is needed for Angry Bots (hence you have to set it to none). However, using the Microsoft C# compiler will allow you to use Microsoft specific APIs without the need for plugins - just wrap the code in #if NETFX_CORE/#endif, and it will compile and work just fine.