Section IDs are only used on the answer sheet when Total Registration prints the primary answer sheet for schools. The system allows you to set one section ID for a teacher (not one for each period). These are used by the College Board to aggregate scores by section ID so that each teacher can access the scores of just their students and not all the other teachers' students. When there is only one teacher for an exam, there is not much need to use a section ID.
You can configure the Section IDs at the bottom of each AP exam screen when you click Manage AP Exams and then choose the exam to edit.