Current app state. As in not past state. Not future state. There would be no need for the distinction if the state wasn't mutated.
> This doesn't mean that your state itself has to mutate. It's entirely possible to structure a React (and/or Redux) app with fully immutable state, copies of which represent change. (Whether copies are done efficiently, e.g. via persistent data structure algorithms, or naively, e.g. via a deep clone, is secondary).
You are confusing the mutability of a data structure with the mutability of state. You can model mutable state using immutable data structures, but it's mutable state regardless. In fact, that is exactly what the Redux/ImmutableJS model is. If the state of your "immutable" react app were really immutable, you would have a webpage, not an app.