Sentence:  Decretals... 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 <WSD>rescripts</WSD>... 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:  An "ad hoc" reply of a pope to some specific question of canon law or morality, without precedential force, sometimes  inclusive of decretals which serve as precedents in canon law.