These two properties are used to control what happens when the object at each end of the relationship is deleted. When an object is deleted, all relationships to that object are also deleted. In some cases, it is necessary to delete the objects at the other end of the relationship. For example, the object at the other end of the relationship cannot exist without the object that is being deleted.
Setting Delete12 to True indicates that when the object at end 1 is deleted, the objects at end 2 must also be deleted.
As a general rule:
When Min1>0, then Delete12 should be set to True.
When Min2>0, then Delete21 should be set to True.