When you sit your HSC exams you will usually get some mark between 0 and 100.
But the thing is, each year the exams vary in difficulty so an 87 in 2017 may not be the same as an 87 in 2018. And obviously, an 87 in Maths Ext. 2 is very different to an 87 in Maths General 2.
So what NESA does is they align the marks for each student to a common scale (90-100: Band 6, 80-89: Band 5 ... ) so that they may be better compared not only across years, but also across subjects.
This is done by judging what a Band 6 (or Band 5 etc.) student should have gotten in the exam.
e.g. for one paper it may be decided that a Band 6 student (on the borderline of band 6) should have gotten an 86. This is the band 6 cutoff. So anyone who gets a mark of 86 would have their HSC mark aligned to 90, which is the borderline of band 6 on NESA's common scale.
Marks of 0 and 100 don't change. If you get a mark that is not on the cutoff it is interpolated linearly. e.g. from the example above, a student who got 93 raw (which is halfway between 100 and the band 6 cutoff) would have their mark aligned to 95.