Self-assessment obligation: pay tax after accounting for deductions and credits, with interest and default consequences applying. Self-assessment requires payment of tax due on the return after accounting for prior tax paid, tax deducted/collected at source, specified foreign tax reliefs and allowable tax credits, with proof of payment to accompany the return; interest for late filing and for shortfall in advance tax is computed on the declared tax reduced by prescribed items, paid amounts are deemed payments toward any subsequent assessment, and non-payment results in the assessee being in default with consequential statutory provisions applying.