Which namespaces of C# can I use for writing xamarin packages compatible with all OSs? : I want to create some nuget package that contains API compatible with all the OSs (UWP, iOS, Android), in order to use it across all the device that can run Xamarin apps.

I heard that the 

> System.*

 namespaces are the ones compatible with them all… is there any other namespace that is universal?

I think that the
> Windows.*

namespace is the one compatible only with UWP, right?

Would like to have a detailed explanation of this topic.

Thank you