Answering in terms of legal studies according to the study design (since I assume that's what you're interested in), it does work almost the same way for all subjects, except that in some subjects SACs are worth less to your study score.
For legal, your Unit 3 SACs are out of 100, and are worth 25% of your study score. Each SAC is worth a certain percentage of this 25%, which differs for each SAC - for instance, a 10 mark SAC on half of unit 3 outcome 1 is worth 10% of this 25%, i.e. 2.5% of your total study score.
This means that the outcome 2 SAC has far more impact on Unit 3 SAC marks, as it's worth 50 marks (1/2 of it, i.e. 12.5% of your SS).
So yes, all your SAC marks are added up to something out of 100; if you lose 1.5 marks on a SAC, but get 100% on all others, your GA mark will be 98.5.
However, recall that this will later be moderated based on your ranking and cohort's performance on the exam - the specific marks aren't important.
EDIT: Where to put this? Probably under the 'Victorian Education Discussion' board.