When a class has a capacity set, you have an option to allow students to be added to a waitlist when registering from the online public registration site. This will allow class to reach full capacity, in case a student drops out of the class or transfers into a different one at any time. When this happens, new students can then be added from the waitlist.
First, you want to ensure waitlists are turned on.
Go to Configure>preferences from your left navigation menu, and within the "preferences" box, see the first option which reads "Allow Waitlist." If it is currently set to "no," click the edit button and change this to yes.
Once this is set, the waitlist becomes automatic. If a class has a capacity of 10, the waitlist will typically begin once someone registers for the last spot.
It is not uncommon, though, for a class with a capacity of 10 with only 9 enrollments to start a waitlist, especially if your online registration site typically sees a lot of traffic during registration periods.
If the class is near capacity, and multiple people add the same class into their cart, it will add the last person to the wait list. Say there are three spots open and four people put the class into their cart at the same time, the fourth person will be automatically wait listed, even if the other three don't end up registering. We currently do not have a workaround for this, because it is the system's way of making sure people are put in the class in order of when they chose it. If they end up not registering the class will be removed from their cart after about a 20 minute window of inactivity. You would simply need to go through and change the wait listed student's status to enrolled if you notice a wait listed student with open spaces left in the class.