The name of the relation that defines the unique or primary
    key definition which is referred to by a foreign key of this
    relation. If there are no referenced-field-names specified with
    this relation-name, then the referenced-relation must have an
    associated constraint which specifies a primary key. If there
    are referenced-field-names, the referenced-relation must have a
    unique or primary key constraint defined which specifies a list
    of unique-field-names. These names have to be the same names as
    in the referenced-relation.