§1. The cardinals especially assist the supreme pastor of the Church through collegial action in consistories in which they are gathered by order of the Roman Pontiff who presides. Consistories are either ordinary or extraordinary.
§2. For an ordinary consistory, all the cardinals, at least those present in Rome, are called together to be consulted concerning certain grave matters which occur rather frequently or to carry out certain very solemn acts.
§3. For an extraordinary consistory, which is celebrated when particular needs of the Church or the treatment of more grave affairs suggests it, all the cardinals are called together.
§4. Only the ordinary consistory in which some solemnities are celebrated can be public, that is, when prelates, representatives of civil societies, and others who have been invited to it are admitted in addition to the cardinals.
Source: Code of Canon Law (Vatican.va)