Literature reference "HZ: (A6.14)" for Jacobian in calibration.cpp

I am interested in learning more about the details of the calibration implementation. I started reading and analyzing modules/calib/src/calibration.cpp (current 5.x branch), which is unfortunately not very verbosely commented, but in line 524 contains the following reference: “see HZ: (A6.14) for details on the structure of the Jacobian”.

I was not able to find out what work it referred to. Can anyone more knowledgeable please decrypt that abbreviated reference?