"What is important with SACs is not the percentage but your ranking in relation to the cohort."
WHAT DOES IT MEAN?
It means that your ATAR is not
directly determined by how well you do on your SACs and end of year exams, but rather how well you do in comparison to everyone else in your cohort. Everything in VCE is dependent on rankings, including your study scores and ATAR. Receiving a 99.95 doesn't mean you got full marks on your SACs and exams, it just means that you did better than 99.95% of the cohort.
Same for SACs, if you get 70% on a SAC, but managed to outperform everyone else in your cohort, then you're rank 1, which typically matters more than what score you specifically got as each school has different SACs anyway.
Even if I get an 90% but I am ranked 7th I won't get a good atar?
Not necessarily. Assuming you're talking about your SACs here, it depends on how strong your cohort is (and also how many people are in your cohort, rank 7 in a class of 12 people is very different to rank 7 in a subject with 100 kids enrolled at the same school). If you're ranked 7th in a subject at your school, and everyone in your school does really well in the final exam, everyone's SAC marks go up and vice versa.