Princess Charlotte is the daughter of the future King of England, Prince William. Like most of the members of the royal family, as she starts to grow, her titles will also change.
Royal title
Princess Charlotte’s royal title was given to her on the day she was born, and she is Her Royal Highness Princess Charlotte of Cambridge. Her Royal Highness is a title given to certain members of the royal family that marks their ranking in the royal family, like monarchs and their consorts use the “Majesty” style in front of their title to mark their ranking. Princess Charlotte also uses “Cambridge” due to her parents Prince William and Kate Middleton being the Duke and Duchess of Cambridge.
Princess Royal
Princess Royal is a title given to the eldest daughter of a monarch. Since Princess Charlotte is the eldest daughter of Prince William, when he becomes King of England, Princess Charlotte will most likely obtain that title. Princess Anne is now holding the title Princess Royal, and even if Prince William comes to the throne, Princess Charlotte won’t have that title until the day her aunt, Princess Anne passes away.
Duchess
Since Princess Royal is a title that has a higher rank, she will most likely not get the title Duchess if she becomes Princess Royal. Dukedoms are for sons and grandsons of a monarch, and for a female to get a dukedom, she must marry a duke. That means that Princess Charlotte will probably won’t get the Duchess title unless The Crown decides to change up some rules, which have already happened in the past. With new law about who gets the right to the throne, Princess Charlotte is now fourth in the line. In the past, when a male is born in the family, he would surpass the female member of the family in the line for the throne, but with this new rule, Princess Charlotte remains fourth, and Prince Louis is fifth in line.
Source: cheatsheet.com