Garb worn by clerics.
All clerics are obliged by law to wear becoming ecclesiastical garb in accordance with legitimate local custom and the prescriptions of the Ordinary of the place.
According to the Code all priests are bound to wear the cassock while they are celebrating the Sacrifice of the Mass and conferring the Sacraments in the Church.
The Third Plenary Council of Baltimore decreed that all clerics, while in the church or at home, should wear the cassock; when, however, they are outside they should wear a shorter black garment coming to knee.
Custom has sanctioned the wearing of the sack coat.
The Roman collar is also part of the clerical costume to be worn abroad as well as at home or in the church.
Minor clerics who do not wear the clerical costume are to be admonished by the Ordinary, and if a month elapses before they resume it they are automatically reduced to the lay state; major clerics in similar circumstances lose any office they may hold and are to be suspended by the Ordinary.
Laymen may not wear a clerical habit unless they be seminarians or aspiring to Orders, or if they are in the service of the Church and are actually taking part in some service either in the church or outside of it.
New Catholic Dictionary