When it was created in 1947, the UN General Assembly charged the International Law Commission (ILC or "the Commission") with "encouraging the progressive development of international law and its codification." The Statute of the ILC expands upon the meaning of this dual mandate. Article 15 explains that the phrase "progressive development of international law" refers to "the preparation of draft conventions on subjects which have not yet been regulated by international law or in regard to which the law has not yet been sufficiently developed in the practice of States." "Codification of international law," by contrast, is said to mean "the more precise formulation and systematization of rules of international law in fields where there has already been extensive State practice, precedent and doctrine."
Bhabha, Jacqueline. "A Progressive Development, Children's Rights and the ILC Draft Articles on the Expulsion of Aliens." Harvard Human Rights Journal 30 (Spring 2017): 15-28.