This is the first round that uses atars. For each course, the uni will look at how many places they have and how in-demand the course appears to be. Then they make offers to how ever many applicants they want to based on atar. Say for a given course, the uni has 100 places available (for simplicity). They, ideally, want to have the applicants with the 100 highest ATAR's in the course, so they make offers to them. Often, unis will actually make extra offers bc they know some people will reject it or get an offer to a higher preference later. So now they have made 100 offers to 100 applicants. Say that 20 of them say no to the offer. Now for the next round, the uni has 20 more places to offer. This keeps going for a number of rounds so that unis can fill their courses and students can get numerous options. So the ATAR required for a course changes every year depending on demand and how many people the uni wants to let in.
For students, you can change your preferences between rounds. E.g. if you get your second preference in the first round, you could still theoretically get an offer for your first preference in the second round if enough places open up. Or if you do get your first preference, you could remove that from your uac course list so that you can get another offer in the second round. So basically each uni makes offers over a number of rounds so that they can control the entry to each course
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