Assessee in default when failure to pay or notify required advance tax instalments or to pay on estimated income. A person is deemed an assessee in default if they fail to pay an advance tax instalment due under an Assessing Officer's order, fail to send the required intimation to the Assessing Officer before the instalment's due date, or fail to pay advance tax based on their estimate of current income, with default applying specifically to the instalment(s) affected by the omission.