Teachers have the option to get notified when students make edits to their response after the work has already been graded.
Notifications will be sent out only for answers that were edited after they were manually graded by the teacher, and they exist to indicate to the teacher that he/she may need to go back and change that grade based on the newly edited response.
When questions are auto-graded there will be no notifications since the grades will automatically update whenever students edit their responses, thus removing the need for a notification to the teacher (the teacher doesn't need to re-check the test, the system does it automatically).
If you're interested in being notified when you might need to go back and re-grade your students' work, go to your account settings by clicking "My Account" and choosing "settings", then proceed to toggle on the option to get notifications.
If you change your mind, and want to opt out of getting notifications again, go back to your account settings and toggle the option off.