Yeah, your final marks for each individual subject will depend on two factors:
- Your examination mark in the actual HSC exam
- Your performance in school assessments (school assessment mark)
Your final mark for a subject will be the average of the two marks.
Most confusion tends to arise from the school assessment mark. Your school assessment mark has almost no affiliation with your actual mark from assessments your school gives out but rather is related to your rank in your class and the percentage gaps between your rank and whoever is first place.
If you are coming first in a subject, then you will receive the highest examination mark received by anyone in your class for the HSC as your school assessment mark. So if the highest exam mark scored in the HSC exam for a particular subject in your class was 94, then the school assessment mark for first place would be 94.
If you are second or below, then NESA will give you a school assessment mark that will be similar to the percentage gap between your rank and first place. So if someone was second place and was 3% off first place, then they might receive an assessment mark that is 3% off the highest exam mark achieved in your class for the HSC exam.
This isn't always the case though, and I have some more detail about school moderated assessment marks over in this post
http://community.boredofstudies.org/461/atar-hsc-marks/385351/ranks-gaps-between.html
Basically, school assessment marks can be a bit of a 'wishy-washy' topic and some people tend to be rather worried about it (including myself back in Year 12). However, as long as your are ranked relatively high in your class and that your cohort is strong in the subject, then school assessment marks shouldn't matter too much.
Good luck!