The Developer Pack is more than just the basic runtime. It's a full suite that includes the , the targeting pack , and the runtime itself. While the runtime allows users to run applications built with .NET 4.5, the Developer Pack enables you to build them, offering the necessary reference assemblies, headers, and libraries in your development environment.
C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.5 net framework 45 developer pack link