Value of taxable supply defines GST liability; transaction value includes specified charges and restricts discounts under invoice rules. Value of taxable supply is the transaction value-the price actually paid or payable when supplier and recipient are not related and price is sole consideration-with specified inclusions (separately charged taxes and duties other than certain Acts, amounts paid by recipient but liable by supplier, incidental expenses, interest/penalties, and subsidies linked to price). Discounts recorded on invoice or documented post-supply discounts with reversal of input tax credit are excluded. Valuation rules or notified categories apply where transaction value cannot be determined; related persons are broadly defined.