Appeal admissibility: appeals require pre-filing tax or advance-tax payment, with limited discretionary exemption by the appellate authority. The amendment revises the start of the limitation period for appeals about assessments or penalties to the date of service of the notice of demand, excludes the period while a section 146 reopening application is pending, and adds that no appeal will be admitted unless the appellant has paid tax due on a filed return or, if no return was filed, an amount equal to the advance tax payable, subject to discretionary written exemption by the Appellate Assistant Commissioner.