The Portable Class Libraries (PCL) in .NET are powered by an API contract, referred to as the "PCL Reference Assemblies".
Microsoft created various profiles of those reference assemblies, one for each use case (For example: "APIs in both Silverlight and Xbox" or "Windows Store and .NET 4.5″).
These API contracts have historically been part of Visual Studio so users on Mac and Linux could not get reference assemblies. This means that they were not able to produce libraries that could be consumed in runtimes that supported them.
We are happy to see Microsoft released those API contracts for developers that want to create binary assemblies on platforms that do not have Visual Studio installed (Mac, Linux or even Windows without VS installed). These are great news for all .NET developers on all platforms, as we now can use the exact blue-print needed to share code across all platforms.
This allows tools like Xamarin Studio on Mac and Linux to produce portable libraries that can be used anywhere .NET or Mono are supported.
Read more: Xamarin blog