Hidden Rank

Hidden Rank is a term used by the Bene Gesserit Sisterhood to indicate that an individual's true rank within the Sisterhood could not be publicly identified.

Known Bene Gesserit of Hidden Rank include the Lady Anirul Corrino, wife of the Padishah Emperor Shaddam IV, and her daughter Irulan Corrino, titular wife of Padishah Emperor Paul Atreides.

Behind the Scenes
Frank Herbert never expanded on why it was necessary for someone from the Bene Gesserit Sisterhood to have their rank hidden. It was possible that the Hidden Rank was used when a person's standing within the Sisterhood was so high that it was necessary to keep it secret, especially if one was married to the Emperor. It is possible that a custom developed for the Imperial Consort to have her rank hidden, as a Reverend Mother might be seen as a threat, or an ordinary Sister seen as a slight.