In my opinion the fixing of salary scales relates to the the POST and not the PERSON. Therefore the whole process should be open and above the board. None of it is confidential, not least that the salary is publicly available when the post is advertised.
There are clearly confidential aspects of employing people, for example grievance and disciplinary matters, appraisals etc etc, but that they are entirely separate from task of setting salary level for a post in the public sector.