What does the title of “Supernumerary Apostolic Protonotary” mentioned in the motu proprio refer to?

Questions and answers about the Motu Proprio “Ad charisma tuendum" and the Extraordinary General Congress of Opus Dei.

The figure of the Prelate receives an honorary title and position that, while reaffirming his secular condition (which is central to the charism of Opus Dei), unites him in a special way to the Holy Father, as part of the so-called “pontifical family.” The title “supernumerary” distinguishes him from those who are notaries in the Holy See.