How are you supposed to interpret this table? What's the difference between scaled and HSC?
Well HSC is the mark you get from the Board of Studies, and the scaled mark is what UAC does to make your ATAR. The scaled marks are then added and then give you an aggregate which is out of 500. To get an ATAR of 90 for last year, you needed about 365/500.
You can use this
atar calculator to figure it out which is easier than the table. For example, you put in a 90 HSC mark for English Advanced, the percentile is 89, which means you came in the top 11% of the state, and the scaled mark is how much it scales down to per unit. So since English Advanced is 2 units 42.1*2 = 84.2. This means a 90 in English Advanced is worth 84.2 scaled. Then if you put English Standard, the scaling is basically the same... but look at the percentile, to get 90 in standard you need to be in the top 0.7% of the state. Play around with it