is a set of reference assemblies that allows developers to build applications for specific .NET Framework versions without requiring that specific version to be the primary runtime on the development machine. Super User Reference Assemblies
Imagine you have Visual Studio 2012 (which uses .NET 4.5 by default) installed on your machine. However, your company has a critical application running on servers that only support .NET 4.0. You cannot simply upgrade the servers, but you need to write code that runs on them. microsoft .net framework 4 multi targeting pack