|Sometimes, in a given relation many Attributes (or sets of them) have all the properties to be a Primary Key . Given that only one of them can be the Primary Key, the other ones are called Candidate Keys. |
Ex.: in a Customer relation, if CustomerNumber is the Primary Key, SocialSecurityNumber and CustomerName are Candidate Keys.
In GeneXus to enforce the uniqueness of the Candidate Key define an User Index with the Unique property.