If both screenChanged and layoutChanged notifications signal changes in the UI and allow you to move VoiceOver's focus somewhere else, what's the difference? To the user, screen changed plays a sound indicating them they got moved to a new screen.

One example from the YouTube app shows that, when VoiceOver is on, and double tapping with the more actions button selected, VoiceOver plays a sound indicating that the screen changed (that's the difference between screen changed and layout changed notifications), and will then move the focus to the dismiss button of the presented screen. The second example is the camera app. When the camera controls button is focused, VoiceOver says: