Although everyone is correct in saying, as i have said, that the cut off isn't predetermined but rather reflects the UAI of the student who had the lowest UAI of those accepted into the course, some do set a kind of b enchmark, which is why also, unpopular courses with heaps of vacancies don't always have a lower UAI in late rounds.
EG nursing at some uni's is 60UAI, and even though there is almost always vacancies after late rounds even, they dont lower this as it has become accepted that 60UAI is the lowest UAI acceptable for acceptance into a university course.
Same with some science courses - they maintain a high UAI, and some even only accept a handful of people, as the uni believes that a person with a lesser UAI would not be capable of successfully completing the course so it wouldnt be worthwhile accepting them