CIRX is the Command Instrument Rating theory exam.
You need to pass it to do your Command Intrument Rating (in an aircraft) in order to attain an Aussie ATP.
The Aussie training syllabus is a bit unique in this regard as I haven't seen this type of knowledge requirement in Africa or Europe.
It forces you to understand the practical aspects of the AIP, flight planning, such as alternate planning, all aspects of IFR planning, MORAs MSAs, MDAs DAs, fuel planning etc, Met, Radio Aids, Flight and Duty regulations, Airlaw etc
It follows ICAO but with the odd Aussie twist here and there it can be very tricky.