Tax invoice requirement: mandates timing, alternatives and exceptions for supplies under GST, including vouchers and continuous supply rules.

Section 31 mandates issuance timing and form of the tax invoice: for goods, before or at removal or delivery; for services, within a prescribed period before or after provision. It permits revised invoices after registration, exempts small-value supplies from invoicing, requires bills of supply for exempt or composition supplies, and requires receipt and refund vouchers for advance payments. Reverse-charge recipients must invoice on receipt and issue payment vouchers to unregistered suppliers. Continuous supply rules prescribe invoicing at each statement, payment, contractual due date, or event completion.

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