NatterType types wherever your cursor is, so it works in every app you already use, on day one, with nothing to set up. If you can type there, you can natter there.
A slice of it. The real answer is "any text field on your Mac".
App-by-app integrations chase APIs that change, and there's always the one app you need that isn't supported. NatterType works at the keyboard, so your niche tool and next year's new app are already covered.