How to classify a structured data relationship when the comparators exhibit multiple relationships

Question

I'm trying to pin down how to understand what defines whether two items in a relationship have a certain type of relationship.

For example, in the following table, Author CAN have a many-to-one relationship with Book. But it doesn't always, and in most instances does not. Would I say Author has a one-to-one or a one-to-many relationship with Book?

Relatedly, is a relationship type determined by which relationship type within scope is most common, or is it determined solely by whether a specific relationship type CAN occur?