Value of supply governs taxable value under GST; transaction value rule sets inclusions and exclusions for valuation. Value of supply is the transaction value-the price actually paid or payable when supplier and recipient are not related and price is the sole consideration. The value includes specified taxes and charges (other than central/state GST compensation levy), amounts incurred by recipient on supplier's behalf, incidental expenses (commission, packing), interest or penalties for delayed payment, and subsidies directly linked to price (included in value of the subsidy recipient). Discounts recorded on invoice or established by prior agreement with reversal of input tax credit are excluded. Prescribed methods apply where transaction value cannot be determined and government-notified supplies follow prescribed valuation.