There is a third, OrderItem, table derived from Item one record at a time. These records are what go into the shopping cart.

     Each OrderItem record has two additional fields: Quantity and Extended Price (Quantity x Unit Price). Extended Price is computed automatically when Quantity is entered; that’s yellow box number 1. So Quantity is a field of a domain object that has behavior.

     Numbers 2 and 3 are computed automatically in the shopping cart. These are behaviors of the OrderItem record itself.