Deferred constraints are constraints for which the Deferred property (the Deferred field of the Constraints window) has been set to Commit or Post Commit. Deferred constraints are not evaluated until commit, or until post commit. Constraints may only be deferred if they comply with the following requirements:
You should defer a corrective multi-record constraint if you know that the correction need not be executed for each manipulated record. This way you will improve application performance.
For example, suppose you have a complicated price calculation constraint involving many tables. By deferring this constraint until commit, you can avoid unnecessary price re-calculation on every manipulation on any of the tables mentioned in the constraint.