Attribute is an instance of the attribute type and has a value.
This value is fixed and unique for every given instance of the attribute type.
Attributes can be uniquely addressed by their type and value.
Instance of data of an entity type, representing a standalone object
that exists in the data model independently.
Entity does not have a value. It is usually addressed by its ownership over attribute instances
and/or roles played in relation instances.
Relation is an instance of a relation type and can be uniquely addressed by
a combination of its type, owned attributes and role players.