Basically you get your external HSC mark (from sitting the test at the end of the year) and that counts to half of your final mark for that subject. The other half comes from your internal performance. Usually (this is a very dumbed down simplified version), your internal mark will be the x highest external HSC mark from your cohort, where x is your ranking from the internal assessment tasks. So, for example, lets say you got 1st ranking in the cohort in english in internals throughout the year. Your internal mark will be the highest external mark that was achieved by your cohort. If you got the best HSC exam mark in your cohort, you will just get that mark for both external and internal. I hope that kinda explains it, others can correct me if I'm wrong here. (preferably the GOAT Jimmy)