Its definition is analogous to the set concept as occurring in the first axiom of mathematics. A type is an aggregation of distinct properties. A property from the collection is called an attribute of the type. This in contrast with the relational data model where each relation corresponds with a database table. A type is defined by a membership relationship.
Instances are irrelevant.
In practical situations this principle can often be extended. Xplain DBMS supports this flexibility in all modules. New meaningful interpretations can be added at any time. Data model specifications imply validity of certain integrity rules. Each attribute in a type definition is related to one and only one equally named type, while each type may correspond with various attributes in other types.