From BiblePortal Wikipedia

Decretum [1]

(or Decretale) is the letter of the clergy and people of a city, sent to the metropolitan and the comprovincial bishops, signifying the election cf a bishop of their city, whom they required to be consecrated. Gregory of Tours says that in the choice of Mauritius the electors could not come to one decretum. The name is also given to a form to be read by the deacon when a bishop is "designated." The difference between this and the foregoing decretum appears to be that the one was sent by the hands of some official of the vacant see immediately on the election of the bishop; if, thereupon, the pope gave his assent, the bishop became technically designate, and the deacon of his church read the decretale or petition for consecration.