HSC Mark
You have your school exams, and the only thing that matters here is your rank in the class and how many marks the next person above you is, and how many marks the person below you is.
So say the school assessment marks are:
A - 94
B - 93
C - 58
Then the HSC exam results are:
A - 74
B - 88
C - 79
The BoS says:
A - HSC Mark: 88 (ranked first, highest HSC exam result)
B - HSC Mark: 87 (ranked second, very close stuff, and so we'll give him something close to it)
C - HSC Mark: 66 (last in the class, far away, so yeah chuck him here)
This explains it:
http://www.boredofstudies.org/moderate.php
Basically they map a curve with the top HSC mark downward as the top school assessment mark and then plot the students on that curve.
Scaled Mark
This takes the average of your HSC mark and your HSC actual exam mark (which you never see).
So for Student A that would be 84, as (88 + 74)/2 = 81.
He may have gotten a high HSC mark but his exam performance let him down.
For Student B: 87.5 (as his HSC mark and exam performance are really good).
Now for scaled marks, they are out of 50 - as a 2 unit course is out of 100. So a 2 unit course is two lots of 50 marks. So if you get 40/50 for Maths, you get 80/100 for Maths as it's a 2 unit subject. So they just count it twice.