I received a written job offer letter from a University in Australia. At the top of the letter it says "Private & Confidential". Can I show this letter to a mentor to ask advice about particular details in the offer? Does "Private & Confidential" mean no-one can read it without my permission? Or is it much stricter, to potentially protect the employer, meaning no-one other than myself and the employer can read it? Nowhere in the letter is the phrase further elaborated on.
IANAL, but all job offer letters should be considered private and confidential, at least it means the following (confidential could mean much more, depending on the exact circumstances):
Private: do not put it anywhere where you personally do not have formal control over it (Obvious for a job offer)
Confidential: Strictly control the number of people you share this with, and take care that there are no conflicts of interest. i.e. do not share it with any of the following: Employees of competitors (no matter about your relationship to them), employees/Management of the company at which you apply (in case they have their own political stake in it).
I think the main function is to protect against competitors probing the salary.
The meaning (and therefore strictness) of "Private & Confidential" can vary quite a lot and may well be organisation-specific so we can't give you a definite answer.
More often than not I'd interpret this as "Don't post this publicly, don't show this to other companies, don't scan this and post it on Glassdoor etc" and I wouldn't have thought showing it to a personal mentor (unless there were some conflict of interest) to be a problem. If you're concerned though you'd be best off asking the person who gave you the offer.