If CASA has no existing regulation to allow, or prevent, something from happening, they write an instrument. At the top of each instrument is a mention of by which regulation they can make this new rule. It will say something like "I Prof. Hubert Farnsworth make this Instrument under CAR 5(a)2(d) iv(a) para 13.b(f)........."
So you go look that ref up, and it will say something like "you cannot do this thing, unless otherwise approved by CASA." The Instrument then becomes your permission to do that thing.
Similarly, Instruments have been written to prevent certain things happening which would normally be allowed, but for some particular reason, this thing happening at this place is a bad idea.
When the regulatory reform program concludes (any day now, surely...) there will be no need for any of these Instruments, as the new fangled "CASR" regulations will be clear, concise, and written in plain English. Every possible aviatory scenario will be covered, either allowed, or not allowed. It'll be awesome, just you wait and see.