§1. A coadjutor bishop and an auxiliary bishop who are not prevented by a just impediment are obliged to perform pontificals and other functions to which the diocesan bishop is bound whenever the diocesan bishop requires it.
§2. A diocesan bishop is not to entrust habitually to another the episcopal rights and functions which a coadjutor or auxiliary bishop can exercise.
Source: Code of Canon Law (Vatican.va)