It's important the students DigiExam client is updated to version 12.1.0 to be able to use the locked question sequence.
If you choose Locked question sequence (MEQ) it means that the student cannot go back and change their answer on a previously answered question.
The student will get information when they start the exam that this is a locked question sequence exam and they will also be notified regarding this during the exam.
When the student has answered a question and choose Next question they cannot go back and change their answer on the previously answered questions. The student is able to see the question title in the left menu.
To see how this looks for the students in during the exam, see this article: Locked question sequence