This setting is used for at-home assessments in which you may submit your progress and return later to modify the submission (e.g., answering more questions or adding additional information to the submission). The Summary page will show the following message and a Submit and return to Dashboard button: 

submitted message

After submission, you will see a confirmation on the dashboard that you have submitted, but there will also be a “Start Test” button, allowing you to re-enter and modify the work you have submitted: 

submission message

If you have NOT submitted your assessment by the test end time, a message will appear on your screen:
not submitted message

Your assessment will auto-submit at the test end time.