It is usually reserved for important events at the Vatican — such as private audiences, canonisations, beatifications and special masses.
The small group of women who have the privilege of the white includes Queen Sofia of Spain, Queen Paola of Belgium, Grand Duchess Maria Teresa of Luxembourg, Queen Mathilde of Belgium, Queen Letizia of Spain, Princess Marina of Naples and Charlene, Princess of Monaco.
The British Royal Family are not allowed to wear white to meet the Pope, as the Queen is the head of the Protestant Church of England.
Princess Charlene wore white to two previous meetings at the Vatican, during an audience with Pope Benedict XVI in 2013 and with Pope Francis previously in 2016.