Each stage can only be associated with a single entity.
The first stage is locked to the primary entity, the entity you selected when creating the business process flow. You cannot change the primary entity after you have clicked on Save.
However, you can add entities other than the primary entity to the business process flow. You should specify the entity for the stage before adding any steps, as subsequently changing the entity on a stage will invalidate the steps in the stage.
If you select an entity that is different from the entity in the previous stage, then a relationship dropdown will appear. There is no restriction as to which entity you can select. A relationship does not need to exist between the two entities.
If there is a one-to-many (1:N) relationship available, then you should select that relationship. In this way, the business process flow will respect...