I don’t know how things work “behind the scenes” but here’s an official statement on the matter:
"70. The Auxiliary Bishop.
The auxiliary Bishop, the principal co-worker of the diocesan Bishop in the governance of the diocese, is appointed in order to provide more effectively for the good of souls in a large or densely-populated diocese or for other reasons connected with the apostolate. The Bishop should therefore look upon the auxiliary as a brother and should involve him in his pastoral projects, decisions and in all diocesan initiatives, so that through their mutual exchange of ideas and opinions, they may proceed in unity and harmony in their intentions and endeavours. For his part, the auxiliary Bishop, conscious of his role at the heart of the diocese, should always act in complete obedience to the diocesan Bishop, respecting his authority.
- Criteria for Requesting an Auxiliary Bishop
a) When the needs of the diocese truly require it, the diocesan Ordinary who seeks the assistance of an auxiliary Bishop should make a petition to the Holy See, explaining his reasons. The request must not be motivated simply by considerations of honour or prestige.
b) When it is possible to provide adequately for the needs of the diocese by appointing Vicars general or episcopal Vicars without episcopal character, the diocesan Bishop should do so, instead of requesting the appointment of an auxiliary Bishop.
c) When requesting an auxiliary, the diocesan Bishop should present a detailed description of the offices and tasks which he intends to entrust to the auxiliary, even when it is a case of replacing an auxiliary Bishop who has been transferred elsewhere or who has resigned his office. In this way the diocesan Bishop personally avails himself of the opportunity to evaluate the quality of his episcopal service for the good of the whole diocese. The diocesan Bishop should not entrust the auxiliary Bishop with the care of a parish nor with tasks of a purely marginal or occasional nature."
vatican.va/roman_curia/congregations/cbishops/documents/rc_con_cbishops_doc_20040222_apostolorum-successores_en.html#Chapter_IV
I don’t know if it can ever be said that an auxiliary is “required” but that is the term used above…
Dan