When creating a new course there is a check box:" Allow Overbooking"
What does it mean? It means that user can subscribe even if the maximum number of students is reached?

Hi,  if the option is unchecked, when there are no more subscriptions available, a student that wants to subscribe will see the button "subscription blocked" and the reason will be "course full". If the checkbox is checked, then the student will see a message: "Can join waiting list". This practicly tells the student that there is the posibility of him beeing subscribed by the admin. So there is no functional difference between the 2 scenarios. The difference is seen in the informative message that the student sees. He will either see "course full" or "can join waiting list".

However, if the course has a price and the "sell course" button is checked, then the user will be able to subscribe to a "waiting list" when maximum subscription number is reached"