Sentence:  <WSD>DECRETALS</WSD>... are the answers sent by the Pope to applications made to him as head of the church, chiefly by bishops, but also by synods, and even private individuals, for guidance in cases involving points of doctrine or discipline...  From the 4th century onwards they formed the most prolific source of canon law. <WSD>Decretals</WSD>... ought, properly speaking, to be distinguished, on the one hand from constitutions... enacted by the Pope "sua sponte" without reference to any particular case, and on the other hand from rescripts... which apply only to special circumstances or individuals, and constitute no general precedent. But this nomenclature is not strictly observed. 
Meaning of the word marked with <WSD> tags:  A papal decree, "particularly" one derived from an ecclesiastical letter.