There doesn't have to be SIS IDs for sections, but you cannot enroll someone into a section using a SIS import unless there is. You can enroll students into a course or into a section. Some choose to enroll in a course, we choose to enroll in a section and that makes it easier if you ever have multiple sections in the same course. SIS IDs have to be unique for that field, but you can reuse a SIS ID in a different field. For example, we have course SIS IDs with the same value as section SIS IDs.
Anything that you want to control through your student information system should have a SIS ID on it. Here's a list of some things that support SIS IDs: account_id, course_id, group_id, group_category_id, integration_id (for users), login_id, section_id, term_id, user_id
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