When adding a new address to the address book, the first tab of the dialog box prompts you to enter a 'Unique' and 'Real' Name for the entry. (N.B. in PPv4 the Unique Name was called a Nickname).
The first field (Unique name) can be used to describe exactly who the contact is (e.g Barclays Bank, Exeter) so you can differentiate between them and other branches of Barclays in your address book.
But if you were writing to them using Probate Plus you might not use that name as you might revert to something more generic. e.g. Barclays Bank in the address field on letters, etc. In that instance then the 'Real' name would be used and take precedence.
If you leave the 'Real' name blank when you set an addressee up, the 'Unique' name will be copied in there.