From memory, you need to know up to about 1800 degrees C on the diagram, and be able to explain what is happening as it cools or heats up at various points in that section. This means the section from the origin up to the solid austenitic range and out to about 3 or 4% where you start getting cast irons.
So basically you want to know all the solid-solid phase transformations which all involve heating to austenite and cooling back down to pearlite, ferrite, cementite, iron carbide or more commonly a mixture of these. It's best to know what these are, at least vaguely how they form, and how to represent them in a diagram. If I remember correctly, a few HSC past papers included questions that involved drawing the grain structures of a number of different steels containing different concentrations of carbon, predicting what proportions of the steel would be pearlite, ferrite, cementite or iron carbide.
I think that there was also one past HSC question that asked for you to explain how the grains of pearlite (which are a lamellar structure of alternating layers of iron carbide (Fe3C) and ferrite form) form as the steel cools down from the austenitic range. You'll need to know the critical temperature of the transition as well (not just for explaining the phase diagram, but heat treatment of steel also - naturally, involves this.
My teacher seemed to really just stress learning the areas around the eutectoid (which is the 0.88% section of the graph), where austenite will cool and undergo a eutectoid transformation completely into pearlite. However, he also liked to go really in depth on every single topic that we covered on the grounds that if we knew more we could produce better answers. So we tended to learn stuff far beyond the syllabus requirements.
In any case, if you're looking for in depth information on the phase diagram, if you could lay your hands on a copy of Introduction to Materials Science by Schlenker (not sure if it's still in print lol), there are a few pages in that book that cover it to a far more than needed, but very clear and understandable level.
