This is the 6272nd most frequent German word.
diözesan
"Diözesan" means "diocesan," relating to a diocese.
The word "diözesan" here is used as an adjective describing the library as pertaining to a diocese, a church district.
Die diözesane Bibliothek beherbergt viele alte Manuskripte.
The diocesan library houses many old manuscripts.
In this case, "diözesan" characterizes the representative as affiliated with a diocese, indicating their area of ecclesiastical authority.
Er ist der diözesane Vertreter bei den nationalen Treffen.
He is the diocesan representative at the national meetings.
Here, "diözesan" modifies the word "rules," specifying that these rules pertain to a diocese or are established by it.