You will not get the 16th highest HSC exam mark for your assessment mark. You assessment mark will, however, be the 16th highest, just like in school - your raw school mark is the 16th highest.
Your raw school mark will undergo moderation based on your cohort's HSC exam performance. The mark that they determine through this process is your assessment mark.
If there was ever a simple explanation...: you and every other student will do assessments and at the end, receive a school mark. This is a percentage mark, based on your performance and weightings for each assessment. Now, BOS needs to compare this mark to the school marks of students in other schools. But just comparing that singular mark to another student who did different assessments is unfair and may not be accurate. So, BOS must have a control assessment, some assessment which every student in the state does. This is obviously the HSC exams. But BOS just doesn't take the 16th highest exam mark and gives you that mark. This doesn't reflect your performance throughout the year, it just reflects the cohort's performance in one exam. Therefore, BOS requires some moderation process to fairly allocate assessment marks to students.
In this process of moderation, there are plenty of elaborate and complex calculations. BOS compares the mean/average of your raw school marks to the HSC exam marks for your school cohort. They then give first place the highest HSC exam mark as their assessment mark, and last place, the lowest HSC exam mark (or it may be slightly higher depending on further calculations). This means ALL assessment marks are within the range of the highest/lowest HSC exam marks. No one can achieve a higher or lower assessment mark respectively. This is now where your school performance is vital. Whilst many disregard school performance and just think about ranks, this is where many students misunderstand the system - there's way too much emphasis on ranks, and not enough on actual marks. BOS looks at how much you deviate from the rank above and below you. They factor this into calculating your assessment mark. For example, coming 16th by 5% puts you in a relatively better position than coming 16th by 15%. In both cases, you are ranked 16th, but the former will generally receive a higher assessment mark if all other variables are kept the same. Before moderation, your school marks are relative to your school. When BOS determines your assessment marks, these will reflect your school performance relative to the state. Essentially, you want as many marks as possible, and be as close to the mark of first place as possible.
At no stage does BOS look at your rank, unless you are first or last ranked. BOS merely looks at your mark differential and factors this into a convoluted set of calculations which you do not need to worry about.
To maximise your marks, you and your cohort must perform well in the HSC exams. IF all your HSC exam marks are within 100 down to say 90, then ALL assessment marks and thus ALL final HSC marks are within the range of 100 down to 90.
The calculations for the ATAR are completely different. Again, nothing you need to worry about - just do as well as possible in school and in the HSC exams.