Departments reserve some seats in their most popular classes and use waitlists to help ensure that all students can complete their required courses on schedule.
Waitlists show how many students want to take particular courses. If demand for a class is unexpectedly high, the department may be able to accommodate additional students by finding a larger room or scheduling another section of the class in the same quarter or in an upcoming quarter.
Some courses are required for all students in a major. Other majors may be able to take these courses as electives. We want to ensure that students who must take a course in order to graduate have priority over students who have other options. For students within the same major, it makes sense to give seniors priority over sophomores for required courses, since seniors are expected to graduate this year, while sophomores have two more years to complete the requirement.