Relations, attributes, and domains all exist on the logical level. The relational model deliberately does not say anything about the physical level. Domains ar

Relations, attributes, and domains all exist on the logical level. The relational model deliberately does not say anything about the physical level. Domains are the data types of columns. If a column has the type VARCHAR, then the domain is varchars. If a column has the type "phone number", then the domain is phone numbers. They exist on the same level. If the database does not support user defined domains, then you can be forced to store phone numbers as varchars rather than use a phone number type. But then the domain is varchar, not phone number.