42 Since pastoral needs require more and more that some pastoral undertakings be directed and carried forward as joint projects, it is fitting that certain offices be created for the service of all or many dioceses of a determined region or nation. These offices can be filled by bishops. This sacred synod recommends that between the prelates or bishops serving in these offices and the diocesan bishops and the episcopal conferences, there exist always fraternal association and harmonious cooperation in the expression of pastoral concern. These relationships should also be clearly defined by common law.
Source: Christus Dominus (Vatican.va)