What is the difference between the Yale College Early Registration period and the Add/Drop period?

The Early Registration period takes place prior to the term of enrollment and is intended to be a gauge of student interest in course enrollment and to provide advance notification of permission to enroll in courses that have limited enrollments. It allows departments to manage demand for enrollment, the number of discussion/lab sections and teaching fellows needed, and the size of the room needed.

The Add/Drop period takes place during the week before and the first week of classes. The Add/Drop period is for refinement of the student course schedule and culminates with a final schedule.