@NaturalId

A natural id is a property or a set of properties that would identify an entity uniquely. We can have at-most one natural id defined for an entity. When Hibernate sees natural-id tag in an entity mapping file, it automatically creates unique and not-null constraints on the properties constituting natural-id

Вот здесь хорошо расписано, с примерами https://prasanthnath.wordpress.com/2013/04/24/natural-ids-in-hibernate/