Closed Classes have a specific start date, stop date, meeting dates/times, location, and instructors. Closed classes are the most common type of class.
An On-Demand allows you to schedule a class whenever a student (or group of student) request a class. There is no set schedule for an On-Demand, a student (or group of students) simply request a single class from your studio.
The main feature of an Open Classes is that it does not have an end date. An Open Class has a specific schedule but continues indefinitely until the student drops the class. Open classes are most commonly seen in Music Schools where a student continues with a class, for example, "Private Guitar Lessons" until he/she drops the class.