A non-hierarchical relationship is a relationship between records that are related by a predefined relationship or a business entity record field.
When you relate records with a predefined relationship, the records in the relationship can belong to the same business entity or different business entities. For example, you can create a spousal relationship between two records that belong to the Person business entity. You can also create an employee relationship between a Person business entity record, such as John Smith, and an Organization business entity record, such as Informatica.
When you relate records with a business entity record field, the records in the relationship belong to two different business entities. For example, an organization record can have a field named Founder that refers to a person record.