Just wanted to add a couple of things on top of both replies above.
1. If you add a course that has additional criteria (ie aren’t just ATAR + adjustment entry) after the deadline for the additional criteria and did not do the additional criteria, then you will not be able to get into the course. For example, you didn’t do a performance for that Fine Arts course you decided on after Round 1 of offers, because you added the courses after the performance times were given/ the performances occurred already.
2. If you add a course between offer rounds, be very careful. Not all courses participate in all offer rounds. Some run out of places to offer in the first few rounds, so if you add one that doesn’t participate in subsequent rounds... there’ll be no offers to receive from that course. (I don’t remember if there’s a list of courses participating/ not participating in subsequent rounds, nor do I know where it is if that exists.)