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The key area contains the primary key for the entity. The primary key is a set of attributes used to identify unique instances of an entity. The primary key may be comprised of one or more primary key attributes, as long as the chosen attributes form a unique identifier for each instance in an entity.
An entity usually has many non-key attributes, which appear below the horizontal line. A non-key attribute does not uniquely identify an instance of an entity. For example, a database may have multiple instances of the same customer name, which means that “customer-name” is not unique and would probably be a non-key attribute.
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