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 will mono C# compiler will allow them to be referenced by JavaScript and Boo scripts, which, for example, is needed for Angry Bots (hence you have to set it to none). However, using 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.