They pick your best 10 units in terms of scaled marks. There are more than enough threads on Scaling + Aligning of marks, so read up on them. Anyways:
http://www.uac.edu.au/pubs/pdf/2006_table_A3.pdf
http://www.uac.edu.au/pubs/pdf/2007_table_A3.pdf
These is the 2006 and 2007 data. It tells you what HSC mark (out of 50) corresponds to the scaled mark that they use in UAI. They use the best 10
scaled marks in your calculations. They'll vary slightly each year, but this gives you a general idea of how much each HSC mark is worth. In 2007, a scaled mark of 79 in Economics was worth 33.2/50, and 85 in Music 2 was worth 32.9/50, which is why Economics was counted ahead of Music 2.
http://www.uac.edu.au/pubs/pdf/2007_table_A8.pdf
This is the data for what aggregate mark the past few years has given what UAI from 2003 to 2007. Now, you can see that if all of your marks are in that range approximately (eg, you get a scaled mark of about 33/50 for your subs, generally low band 5s) you'll probably end up with a UAI above 80, but below 85.
