This feature is conditional on the feature and will only appear if, "Allow students to join multiple events with the same topic" is set to "Not Allowed". The options here are "Allowed" or "Not Allowed". If you set this to, "Allowed" students will be able to join a waitlist for all events, regardless if they are already registered for an event with the same topic/course. For example, if a student signs up for Event A and they want to join Event B, which is for the same topic/course and already has a waitlist, they will be able to join the waitlist.