The International Law Commission is the General Assembly's own organ for codifying international law, and its members sit in their individual capacity
Background
The International Law Commission is a subsidiary organ of the United Nations General Assembly, whose object is the promotion of the progressive development of international law and its codification. The Commission is governed by a Statute concluded at New York, 21 November 1947, and annexed to General Assembly resolution 174 (II) of that date. The Commission began with fifteen members; the number rose to twenty-one in 1956, twenty-five in 1961 and thirty-four in 1981. The term of office of members, initially envisaged as three, was extended to five years as early as 1950. The Statute originally provided that the Commission should sit in New York, but this was changed to Geneva in 1955, and after its first session the Commission has almost always met there. Members are to be persons of recognized competence in international law, and while the Statute leaves the point implicit, it is clear from the negotiating history and from subsequent practice that they serve in their individual capacity, not as representatives of States.
In plain words
Subsidiary organ — A body created by a principal organ of the United Nations to carry out part of its work.
Progressive development — Drafting new rules on subjects international law has not yet settled.