It is often stated on the envelope of bank statements, or certain email messages:
Private and Confidential
Presumably, "Private" and "Confidential" are two different legal terms, hence the need to state both of them. What is the difference?
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There may be jurisdictional differences, but this link gives a thorough summary based on US interpretations.
In short, privacy is an expectation based on common law while confidentiality is an ethical duty - usually (but not exclusively) part of a professional interaction.
For correspondence where the sender wishes to show that they are aware of their ethical responsibility and that they also recognise that the addressee would have an expectation of privacy, both are often written.
In a business environment, a confidential letter could relate either to the personal affairs of the individual employee or to the position that the employee holds. If it relates to the personal affairs of the individual employee, it private as well as confidential.
For example, a letter about the employee's estimated retirement benefits would be private and confidential, while a letter stating that a certain test on a product the employee designed was successful would be just confidential, not private.