If you apply for a job, but all available positions have already been filled, you will be placed on the waitlist. If someone cancels on the job or is not able to make it, you will be offered the position again.
Keep in mind that this may occur on short notice - but you are not required to accept the job again if you are offered a waitlist promotion.
If you are on placed on the waitlist, you’ll get a notification whenever you are selected to actually work the job. You can opt out of the job without penalty if you’re no longer interested or you can confirm the job if you still want it. It is important to realize that you can be on multiple waitlists at the same time, so you can pretty much keep accepting jobs until you’re confirmed for one. Whenever you are confirmed for a job, it will override the rest of the jobs that you are waitlisted for.