§1. With every contrary custom reprobated, the following cases are reserved to a collegiate tribunal of three judges: 1/ contentious cases: a) concerning the bond of sacred ordination; b) concerning the bond of marriage, without prejudice to the prescripts of cann. 1686 and 1688; 2/ penal cases: a) concerning delicts which can entail the penalty of dismissal from the clerical state; b) concerning the imposition or declaration of an excommunication.
§2. The bishop can entrust more difficult cases or those of greater importance to the judgment of three or five judges.
§3. Unless the bishop establishes otherwise in individual cases, the judicial vicar is to assign the judges in order by turn to adjudicate individual cases.
§4. If it happens that a collegiate tribunal cannot be established in the first instance of a trial, the conference of bishops can permit the bishop, for as long as the impossibility continues, to entrust cases to a single clerical judge who is to employ an assessor and auditor where possible.
§5. The judicial vicar is not to substitute judges once they have been assigned except for a most grave cause expressed in a decree.
Source: Code of Canon Law (Vatican.va)