Problem: I would like to know whether I should use a Post Reference field or a Post Relationship, and understand the differences between the two.
Solution: A post reference field is a very simple version of a one-to-many (O2M) relationship. In the database, they are handled identically. The main differences are how they are managed in wp-admin. For example, with a O2M relationship between Clients and Vehicles, you can edit the parent Client post and see all the child Vehicle posts easily in the post relationship editor panel. If you want to disconnect one of the Vehicle posts, you can do it from the Client post. However, with a post reference field, you cannot easily manage the related Vehicles from the Client post. You would have to edit each Vehicle post to change the post reference Client. So if you want more management capabilities in wp-admin, you should use a O2M relationship. If you do not need this type of management, you can use a Post Reference field instead.
If you need the ability to create multiple Client post references in the same Vehicle post, you should use a M2M relationship instead of a repeating post reference field.
