I believe that .NET Core is not a subset of the .NET Framework; e.g. .NET Core might have things that are not yet in .NET framework.

I believe that the things that go into .NET core eventually go into .NET Standard (and thus the framework), but at a slower pace.

Thus it would be:

  .NET ECMA Spec ⊆ .NET Standard ⊆ (.NET Core ∩ Mono ∩ Xamarin ∩ Unity ∩ .NET Framework)