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The set of possible values an attribute can take is the domain of the attribute. When modifying your data and describing attributes, it is important to think of their domains. Each instance of an entity must contain values for all attributes that define the entity.
If an entity contains the attribute City, each instance of the attribute must draw its value from the domain of City. It cannot draw its value from any other domain.
Two attributes may have common characteristics (the same length, data type, and valid values) but not share the same domain. The attributes Customer Number and Order Quantity both may be 6-digit numeric fields. However, they draw their values from different domains, a Customer Number domain and an Order Quantity domain.
Other attributes may have common characteristics and share the same domain. For example, Order Quantity and Shipped Quantity are both 6-digit numeric fields. They share the same domain and draw from the same set of possible values.
In relational modeling, you can compare attributes that share the same domain. You cannot compare attributes that do not share a domain.
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