The procession of the Cardinals into the Sistine Chapel, as well as the actual places where each Cardinal will sit during the Conclave is not determined on a first come first serve basis. As Catholics have come to expect of Holy Mother Church, there is always order to all that she does and no detail seems to escape her attention. So how are the Cardinals seated and what determines their order in a procession?
The Vatican refers to this order of seating and procession as ‘precedence’. It has nothing to do with the size of a Cardinal’s diocese, the importance of his job in the Church, or any of the other things that naturally spring to mind.
There are three orders of Cardinals: Cardinal Bishops, Cardinal Priests and Cardinal Deacons. In seating and procession the Cardinal Bishops lead, followed by the Cardinal Priests and then the Cardinal Deacons.
Cardinal Bishops are the six senior Latin rite bishops who actually work in and reside in the Vatican.
Cardinal Priests are made up of the diocesan bishops who administer dioceses all around the world.
Cardinal Deacons are bishops who work as officials in the Curia.
Within each category of Cardinal, the order is determined by the date and time when the person was named Cardinal. The person who was created Cardinal first goes ahead of those created Cardinal after him.
In those instances where Cardinals were created Cardinal on the same day, this order is determined by whose name the Pope read out first when the list of the bishops, who are to be created Cardinal, was read out by the Pope.
Once again the order in which the Pope reads out the list of persons to be created Cardinal is not done alphabetically or randomly. The order of the list is based on who was ordained as a bishop first. Therefore, even if two bishops may be created Cardinal on the exact same day, the Bishop who was ordained as a bishop first, is always read out first by the Pope.
In summary: In seating and procession Cardinal Bishops lead Cardinal Priests, who lead Cardinal Deacons. Within each, the person who was created Cardinal first, goes first. Those created Cardinal on the same day are sorted by who was ordained as bishop first.