A more thorough explanation:

Diocesan

Diocesan means something that is related to a diocese, which is a district under the care of a bishop. It can also refer to the relationship between a bishop and the clergy within the diocese.

• The diocesan bishop presided over the confirmation ceremony.
• The diocesan council met to discuss the budget for the upcoming year.

These examples illustrate how the term diocesan is used to describe things that are related to a diocese. In the first example, the bishop is referred to as the diocesan bishop because he is the bishop in charge of the diocese. In the second example, the council is referred to as the diocesan council because it is made up of representatives from the various parishes within the diocese.