In this entity-relationship diagram, a customer's phone number attribute is a collection that can contain zero to ten phone numbers.
A customer's address is modeled as a single Address object, which contains the attributes Street, City, State, and Zip. The Address object associated with a customer is also used by any Purchase Order objects associated with that customer. In a Purchase Order, the customer's address is used as the shipping address.
A Purchase Order contains as an attribute a collection of one or more Line Items. Each Line Item refers to one Stock Item.