Patent royalty tax: fixed low-rate taxation with expense disallowance and conditional opt-in consequences and restricted deductions. Royalty income from a patent developed and registered in India is taxable at 10% under a special regime, with no deduction allowed for expenditure in computing that royalty income. An eligible assessee may opt for taxation under this section by the due date under section 139(1); failure to continue to report the income as required in any of the succeeding five assessment years causes forfeiture of the special regime for five subsequent assessment years. A deduction under section 80RRB is available subject to a statutory limit. 'Developed' requires at least 75% of expenditure incurred in India.