You first get a PIN from UAC then you fill in the application form and pay the required fees. From here on you would be able to fill in 9 different choices of degrees which you can select from the UAC Handbook.The 1st choice is the degree and the uni you really want to get in.
They are different rounds which you can get in your preferred course but this all depends on your marks,ranks and etc.But if you got into your first preference you cannot accept your other choices you listed . But for example you got an offer for your 5th preference and you accepted it, you may wait for the later rounds to see if you got in your 1st choice which then you could defer the degree you have accepted at the beginning.