In order for Parent/Child or Join relationships to be properly built, both source and target entities should have an identifying element. Identifying elements are not required when the relationship is first defined. However, an error will result when the entity is built if no identifying element has been defined.
For variation relationships, the source entity must be a data or external entity and must have an identifying element. The target entity must be a variant entity and must not have an identifying element.
For include relationships, the source entity must be a data entity. The target entity must be a conceptual entity and no identifying element is allowed for the conceptual entity.
For a variant entity, a Variation relationship must exist before you can create Join relationships using that variant entity as the target entity. Once the Variation relationship is created, the identifying elements are known in the variant entity.
The elements of an entity are directly impacted by the relationships defined. For more details, refer to 6.2 Elements and Relationships.