1. Your internal marks (the overall marks you get from all school assessments) and internal rank are used as inputs in the Moderation process (done by NESA)
2. The Moderation process output combined with your raw external marks (from the external HSC exams in October of each year) combine equally to form your overall raw mark
3. Your raw marks are used as inputs for the separate Scaling and Alignment processes
4. Scaling is done by the UAC (Universities Admissions Centre). This process calculates the scaled marks you get from your raw mark for each subject. The total scaled marks from your best 10 units (including at least 2 units of English) is called the aggregate and is used to calculate your ATAR, again done by the UAC. ATAR is important (this number gets you into university!) therefore scaling is important!
5. Alignment is done by NESA and this works out your HSC marks. HSC marks are simply an indication of how well you did according to the band descriptions of the course – it does not determine your ATAR (scaled marks do that!) so therefore it is nowhere near as important as scaled marks.
Your Goal as a HSC Student
Your goal as a HSC student is to get into your ideal university course. To do this, you need to make the ATAR cutoff for that course. Therefore you need enough scaled marks! Therefore, in year 10 when you choose your subjects for year 11, make sure you choose high scaling subjects that you think you can do well in.
Do you see the problem with the NSW HSC system? The system forces young impressionable clueless year 10 students to make a massively impactful decision!
Anyway, if you’re stuck with several low scaling subjects, do not fret. There’s nothing stopping you from achieving whatever ATAR cutoff you need. Although, in common experience, it is more difficult achieving higher scaled marks in a low scaling subject given an equal effort input, you can do it.
TL;DR: try your best in all your subjects, focus more time and effort on your weaker subjects.