Referential Constraints provide the Optimizer with an efficient method for producing better query plans that take advantage of defined, but unenforced, referential relationships.
Other than not enforcing referential integrity and not requiring the candidate key for the referenced table in the constraint to be explicitly declared to be unique, the rules for Referential Constraints (Referential Constraints are sometimes called soft referential integrity constraints.) are identical to those documented by Adding or Dropping Standard Referential Integrity Constraints. See Referential Constraints for additional information.