It goes the other way, too. For example, you might decide that—in
the context of your particular app—the selected tab in a bottom
navigation bar is not ephemeral state. You might need to change it
from outside the class, keep it between sessions, and so on.
In that case, the _index variable is app state.

There is no clear-cut, universal rule to distinguish
whether a particular variable is ephemeral or app state.
Sometimes, you’ll have to refactor one into another.
For example, you’ll start with some clearly ephemeral state,
but as your application grows in features,
it might need to be moved to app state.

For that reason, take the following diagram with a large grain of salt:

When asked about React’s setState versus Redux’s store, the author of Redux,
Dan Abramov, replied:

“The rule of thumb is: Do whatever is less awkward.”