Diese Anpassung wird in der Datei project.json vorgenommen.
Answering your question in a different way: A library should target environments which SDK its requires. If you do not require a SDK use dotnet.
So if your library only has some algorithms or is not specific to the platform, use dotnet. If any of your dependencies is restricted, this dependency will propagate up to the application (e.g. DNX, UWP, .Net46) which uses it.
I can only highlight like Malachi the article series of Oren. (he just wrote a new one: https://oren.codes/2015/07/29/targeting-net-core/ on the same topic).
ps: dotnet is not a concrete runtime it is the abstraction of it. It is a target which in this case does not even specifiy a runtime but instead says: Anything which interprets IL correctly goes. For example dnxcore5 is a target which specify a SDK(DNX) which has a specific runtime (CoreCLR). In this case you can make further assumptions about the runtime behavior (like usage of JIT, availability of x-plat implementation, etc.).
pps: be aware that the dotnet name is transformed into the term netplatform with one of the next core and dnx releases (IMHO rc2)