Transaction value determines taxable supply under GST; includes specified additions and excludes prescribed discounts and notified valuations. Section 15 sets the transaction value as the primary measure of the value of taxable supply when supplier and recipient are not related and price is sole consideration, and requires inclusion of specified additions: certain taxes and charges (other than GST and compensation cess) when separately charged, amounts paid by recipient on supplier's behalf, incidental expenses (commission, packing), interest or late payment penalties, and subsidies directly linked to price. It excludes discounts recorded in the invoice or established by pre-existing agreement with reversal of attributable input tax credit. Related persons are defined and alternate prescribed valuation methods are provided where transaction value cannot be determined.