Value of supply: transaction value governs taxable valuation, with specified inclusions and defined related-person rules. Section 15 prescribes the transaction value as the default method to value taxable supplies where parties are not related persons and the price is sole consideration. It mandates inclusion of specified taxes and charges, supplier-liabilities borne by recipient, incidental expenses, interest/penalties and subsidies directly linked to price. Specified discounts are excluded if invoiced or if post-supply discounts are pre-agreed with reversal of attributable input tax credit. Where transaction value is inapplicable, valuation follows prescribed rules and notified supplies are valued as prescribed. 'Related persons' and 'person' are defined, and sole agents/distributors/concessionaires are treated as related.