Value of supply governs GST valuation: transaction value with prescribed inclusions, exclusions, and discount conditions. Value of supply for GST uses the transaction value when parties are unrelated and price is sole consideration; value includes separately charged other taxes, supplier liabilities paid by recipient, incidental expenses, interest or penalties for delayed payment, and subsidies directly linked to price (included for the recipient supplier). Discounts recorded on invoice at or before supply are excluded; post-supply discounts are excluded only if contractually established and input tax credit attributable to the discount is reversed by the recipient. Where transaction value cannot be determined, valuation follows prescribed rules and notified supplies are valued as prescribed. Related persons and legal persons are defined.