§1. The college of cardinals is divided into three orders: the episcopal order, to which belong cardinals to whom the Roman Pontiff assigns title of a suburbicarian church and Eastern patriarchs who have been brought into the college of cardinals; the presbyteral order and the diaconal order.
§2. The Roman Pontiff assigns each of the cardinals of the presbyteral or diaconal orders his own title or diaconia in Rome.
§3. Eastern patriarchs who have been made members of the college of cardinals have their own patriarchal see as a title.
§4. The cardinal dean holds as his title the Diocese of Ostia together with the other church he already has as a title.
§5. Through a choice made in consistory and approved by the Supreme Pontiff and with priority of order and promotion observed, cardinals from the presbyteral order can transfer to another title, and cardinals from the diaconal order to another diaconia and if they have been in the diaconal order for ten full years, even to the presbyteral order.
§6. A cardinal transferring through choice from the diaconal order to the presbyteral order takes precedence over all those cardinal presbyters who were brought into the cardinalate after him.
Source: Code of Canon Law (Vatican.va)