i'm not completely sure but i think that the rank of your school mark...and that's your trial plus assesments is used to place the marks of your HSC into the bell curve. So it kind of means that the marks of your trials in conjunction with the marks of your assesments gives you a class rank. This rank is used, say if the people taking the exam get all really high, this rank will be used to spread people out along the whole marking range. Same if everyone does bad. This is for your HSC mark, your UAI is judged by your mark in individual subjects and the performance of all people in that subject, so if you get a great mark in something like hospotality and lots of peole get shocking marks, your UAI is likely to be draged down, that's why teachers always say its better to do average in advanced English than great in standard. Don't quote me on this cause it's only what i understand from listening to teachers and the career advisor.
Hope this helps.