I’d like to clarify “Sitting in Choir,” since there have been several explanations that overlap.
First thing to know is that the use of the word “Choir” refers to an architectural part of the traditional layout of a Catholic church. As you can see in the diagram below, the choir is the part of the church between the nave (where the people sit, to the left of the highlighted portion in the diagram), and the altar (where the Sacrifice of the Mass is offered, to the right of the highlighted portion of the diagram). When a bishop, priest, deacon, or seminarian “sits in choir,” they sit in this part of the church, usually on special benches called “choir stalls.”
© University of Pittsburg
Why Would a Person Sit in Choir?
The next portion is defining the purpose of someone sitting in choir. Historically, the clergy would sit in this area to say the responses, and chant the antiphons for the Mass, since they were able to read the appropriate texts. Eventually, this became the duty of a choir as we think of it today — a group of laymen who usually occupy the back of the church, and chant antiphons or sing hymns for the Mass.
Despite this evolution, clerics still sat in the choir section of the Church. This is still common in the Extraordinary Form of the Roman Rite, because concelebration is not permitted. Since only one priest could offer the sacrifice of the Mass, other priests sit in choir to make the responses (along with the servers). These priests may assist with distributing Holy Communion by donning a stole over their surplice.
Deacons can also sit in choir when they are not acting as Sacred Minister for the Mass.
Seminarians also can sit in choir, and this can be a valuable experience for them to better learn the Mass, including the actions of the priest.
Is Sitting in Choir Allowed in the Mass of Paul VI?
Shortly: Yes. Several Vatican Documents recognize the validity of the option of sitting in choir, while also recognizing that concelebration is allowed. There are many reasons for this.
Firstly, as bishops cannot concelebrate when a priest is celebrating, a bishop will sit in choir when attending a Mass. This is common when a bishop visits a parish, and the pastor still celebrates the Mass. While sitting in choir, a bishop still presides over the celebration, and will sometimes address the faithful, or give the final blessing.
Priests will sometimes refrain from concelebrating due to various reasons, including them being mindful of the maximum number of Masses they may celebrate a day (which is dictated by Canon Law); or their being mindful that they are in a state of mortal sin, and they do not wish to unnecessarily receive the Eucharist when they are lacking a state of grace.
Deacons and seminarians will sometimes sit in choir to assist, or to show their presence, especially at more solemn liturgies.
I hope that was clear, if anyone has anything to add, please do so.