Imagine that you have a Customer which in turn has two Addresses: ShippingAddress and BillingAddress. These addresses or content items, just like the customer is.
Next, you want to create a content part editor where the user can edit the Customer fields as well as the fields of the two addresses.
Furthermore, the Address has a Country navigation property, where Country is a simple entity class, not a content item.
We want to enable the user to pick one of the available countries using a dropdown list.