There is a beautiful article series about it which solved my questions around it ...

https://oren.codes/2015/06/16/demystifying-pcls-net-core-dnx-and-uwp-redux/ https://oren.codes/2015/07/29/targeting-net-core/

.Net Core has all his libraries (e.g. System.IO) in separate NuGet packages (each of them available for the SDKs DNX, UWP and .Net 4.6). Third party libraries target dnxcore50 (DNX) or uap10.0 (UWP) if they access the platform natively or rely on their features. If they do not access the platform but only rely on other packages, they should target dotnet.

dotnet effectively means: I am compatible with any platform which satisfy my dependencies (your library XYZ "dotnet" which uses System.Reflection dnxcore5+net45 could not be used by a UWP uap10.0 app). This effectively ends the combinatoric nightmare of the platforms. The previous target combination dnxcore5+net45 created an intersection between the platforms libraries and each addition would make the situation even worse. dotnet on the other side does not restrict the library on a target but instead forwards this restriction decision to its dependencies (where suddenly new restrictions like the famous unicorn platform can show up).

Therefore as a library author you can target dotnet if you just need other libraries.

Answering your question:

• Your PCL is compatible with .Net Core style environments like DNX and UWP if you add the target dotnet, dnxcore50 or uap10.0 depending on the need of your library (see Owen's article for same basic compatibility with the contract Profile 259).
• .Net Core is much more than a set of PCLed libraries. It is a new CLR, a new organized framework (packaged in small parts) and the infrastructure for new .Net SDKs (DNX, UWP, and whatever comes next). The term ".Net Core" targets both, the base class library "CoreFx" and the CLR "CoreCLR". But the real platforms are in reality DNX (by ASP.Net team) and UWP (by Windows team).

All of that answer is my current understanding of the .Net Core library situation. It is work in progress, and like mentioned in the posts, not yet public documented.

NOTE DEC 2016: Be aware, dotnet as the predecessor to netstandard1.x has changed in its concept starting with netstandard2.x (.NET Core 2.0; ~JUN 2017). Beginning with netstandard2.0 there will be one common contract (the netstandard.dll) which all platforms (.NET Core, .NET Framework, Xamarin, Mono, Unity3D) implement. This contract will be extended over time and the platform have to either drop support for the latest standard, throw NotImplementedException or implement it.

My understanding is they both are in concept different.

• .NET Portable built upon the .NET Full, .NET Core, Windows Phone,etc, as sort of 'bridging layer'.
• It actually has no concrete implementation, consider it a 'Package' of assemblies of the 'Interfaces'(contracts).
• The scope of .NET Portable 'Package' is dynamic, depends on what 'Targets' platforms you are going to 'bridge'. It's a intersection of the platforms you target, the more platforms the smaller package.
• During runtime, this Portable layer is hooked/adapted to the real implementation of .NET full, .net core or...