25 The pastoral office of Bishops should be so constituted for the governing of dioceses that the good of the Lord's flock is always the supreme consideration. Rightly to achieve this goal, auxiliary bishops will frequently be appointed because the diocesan bishop cannot personally fulfill all his episcopal duties as the good of souls demands, either because of the vast extent of the diocese or the great number of its inhabitants, or because of the special nature of the apostolate or other reasons of a different nature. Sometimes, in fact, a particular need requires that a coadjutor bishop be appointed to assist the diocesan bishop. Coadjutor and auxiliary bishops should be granted those faculties necessary for rendering their work more effective and safeguarding the dignity proper to bishops. This, of course, should always be accomplished without detriment to the unity of the diocesan administration and the authority of the diocesan bishop. Furthermore, coadjutor and auxiliary bishops, since they are called to share part of the burden of the diocesan bishops, so should exercise their office that they may proceed in all matters in single-minded agreement with him. In addition, they should always show respect and reverence for the diocesan bishop and he, in turn, should have a fraternal love for coadjutor and auxiliary bishops and hold them in esteem.
Source: Christus Dominus (Vatican.va)