Unjust enrichment doctrine clarified for provisional assessments; proviso to Rule 9B added and appeal initiated. The Board clarified that the doctrine of unjust enrichment applies to refunds from provisional assessments by simplifying Rule 9B through a proviso added in June 1999; finding the tribunal view debatable, the Board secured legal advice, obtained Attorney General approval, filed a civil appeal, and instructed field formations to act in accordance with the amended provision.