You'll get a piece of paper, iirc - that'll say what units counted. Along with your ATAR.
ohk, so there's no way to find this info out atm online. But tomorrow a letter comes.
EDIT: From the UAC website,
"Which courses count towards my ATAR?
Only ATAR courses can be used in the calculation of your ATAR. Your ATAR is calculated from your aggregate of scaled marks from your ATAR courses, including two units of English and your best eight units from the remaining courses (according to the ATAR rules).
Your ATAR Advice Notice tells you which of your courses were included in the calculation of your ATAR. Just by looking at your HSC marks, it's not possible to tell which courses will be included in your ATAR. Sometimes the course with the lowest HSC mark won't count, but this is not always the case."