Whenever you create an entity in your data model, one of the most important questions you need to ask is: “How can a unique instance be identified?” In order to develop a correct logical data model, you must be able to uniquely identify each instance in an entity.
In each entity in a data model, a horizontal line separates the attributes into two groups, key areas and non-key areas. The area above the line is called the key area, and the area below the line is called the non-key area or data area. The key area of CUSTOMER contains “customer-id” and the data area contains “customer‑name,” “customer-address,” and “customer-phone.”
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