I did my HSC in 2012, and at that time there was like a footnote corresponding to my 4U mark that read, "Mathematics (Extension 2) students are not required to sit for the 2 Unit Mathematics examination. If this student had sat for the 2 Unit Mathematics examination it is likely that he/she would have achieved Band 6 in that course".
So unless it's changed, there's just a minor off-hand comment but otherwise it just shows your marks for your extension Maths subjects.