A more thorough explanation:

Definition: Decretals are canonical epistles written by the Pope or by the Pope and his cardinals to settle controversial matters in ecclesiastical law. They are part of the Corpus Juris Canonici, which is a body of western ecclesiastical law compiled from the 12th to 14th centuries. The decretals consist mainly of four collections: Decretales Gregorii Noni, Decretales Bonifacii Octavi, Clementinae, and the Extravagantes.

Example: The Decretals were used to settle disputes and controversies in the Church, such as issues related to marriage, divorce, and the authority of the Pope. For example, the Decretals of Gregory IX were used to establish the Inquisition, which was a court set up to investigate and punish heresy.

Explanation: The example illustrates how the Decretals were used to settle disputes and controversies in the Church. The Decretals were important in establishing the authority of the Pope and the Church, and they were used to regulate the affairs of the Church. They were also influential in the development of English law, as many of the judges in the royal courts were ecclesiastics who were familiar with the doctrines of canon law.