A post object field and a relationship field are, for the most part, the same thing. They post store the post ID of the related post. The major difference between the two field types is that a post object field only stores a single value by default, which stores a single value in the database, unlike the relationship field that stores an array. If the post object field is set to allow multiple values then it functions exactly like the relationship field. Hope that answers your question.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users