§1. Each diocesan bishop is bound to appoint a judicial vicar, or offcialis, with ordinary power to judge, distinct from the vicar general unless the small size of the diocese or the small number of cases suggests otherwise.
§2. The judicial vicar constitutes one tribunal with the bishop but cannot judge cases which the bishop reserves to himself.
§3. The judicial vicar can be given assistants who are called adjutant judicial vicars, or vice-officiales.
§4. Both the judicial vicar and adjutant judicial vicars must be priests, of unimpaired reputation, doctors or at least licensed in canon law, and not less than thirty years of age.
§5. When the see is vacant, they do not cease from their function and cannot be removed by the diocesan administrator; when the new bishop arrives, however, they need confirmation.
Source: Code of Canon Law (Vatican.va)