When a trip or course is full people can add themselves to the waitlist. There are three waitlist spots as reflected on the registration page.
The waitlist comes in to play when a member registered for a trip or course cancels. When someone needs to cancel they let the leader/instructor know.
If the leader/instructor determines that there is still time for someone to take the empty spot, they can let the website administrator/trips officer/training officer know so the canceled booking can be removed.
Now there is a spot available so the first person on the waitlist will automatically get an email giving them a link to register for the trip within 48 hours or else they can use the link to cancel their request to join.
If they don't do anything within the 48 hours they receive an expiration email.
Then the next person on the waitlist will automatically receive an email saying that the spot is available.
The idea is to keep a transparent waitlist and also keep the leader/instructor in the loop regarding people cancelling and joining last minute.