Procedural validity: GST assessments and related notices remain effective if they substantially conform to statutory requirements. Proceedings including assessment, adjudication, review, revision, appeal, rectification, notices and summons under the Act are not invalidated solely for mistakes, defects or omissions in form if they are in substance and effect consistent with the intents and requirements of the Act or other law. Service of a notice, order or communication is not open to challenge where the addressee has acted on it or where service was not previously contested in earlier related proceedings.