3 The College, which does not exist without the head, is said "to exist also as the subject of supreme and full power in the universal Church." This must be admitted of necessity so that the fullness of power belonging to the Roman Pontiff is not called into question. For the College, always and of necessity, includes its head, because in the college he preserves unhindered his function as Christ's Vicar and as Pastor of the universal Church. In other words, it is not a distinction between the Roman Pontiff and the bishops taken collectively, but a distinction between the Roman Pontiff taken separately and the Roman Pontiff together with the bishops. Since the Supreme Pontiff is head of the College, he alone is able to perform certain actions which are not at all within the competence of the bishops, e.g., convoking the College and directing it, approving norms of action, etc. Cf. Modus 81. It is up to the judgment of the Supreme Pontiff, to whose care Christ's whole flock has been entrusted, to determine, according to the needs of the Church as they change over the course of centuries, the way in which this care may best be exercised—whether in a personal or a collegial way. The Roman Pontiff, taking account of the Church's welfare, proceeds according to his own discretion in arranging, promoting and approving the exercise of collegial activity.
Source: Lumen Gentium (Vatican.va)