world income "world income", in relation to a certificate which is an acceptable certificate, means the amount of any income specified in that certificate in accordance with paragraph (b) (ii) of the definition of "certificate" in this subsection, as reduced by the amount of the depreciation allowances specified in that certificate, less any sums received in the relevant period or prior thereto which are refunded in the relevant period and any sums first, receivable in the relevant period or prior thereto which in the relevant period cease, otherwise than on the receipt thereof, to be receivable; ACT 53 - INCOME TAX ACT 1967 54. Sea and air transport undertakings.