Are they related? I have heard of both
The High School Certificate (HSC) and your Australian Tertiary Admissions Rank (ATAR) are two different things, though they are inherently intertwined.
Your HSC is a set of examinations and assessments that are standardised across all students studying in NSW and some students from other locations around the world (such as Canberra). The Board of Studies (BOSTES) writes the HSC exams every year for students studying the HSC only.
Your final HSC results are determined by your internal assessment mark and your final examination mark. These are sent to your state’s tertiary admissions centre where your ATAR is then calculated using a scaling algorithm that adjusts for the difficulty of subjects.
Your ATAR is a rank, not a mark. To get an ATAR of 99.70 means that your HSC marks were better than 99.7% of the candidature. How the ATAR is calculated changes from year to year. Each of your subjects are scaled according to their difficulty and these scaled marks are used to determine your ATAR.
The reason your ATARs are considered for your university admissions and not your HSC marks is because there are many combinations of HSC subjects that may be taken. Your ATAR standardises these combinations and gives a number that quanitifies your HSC marks.
Everyone student in Australia gets an ATAR, regardless of whether they sit the HSC (NSW), the VCE (Victoria), or other standardised tests for other states. Not everyone in Australia get HSC results, as not everyone sits the HSC.
Hops this helps!