Best judgment assessment applies when a taxpayer fails to file returns or comply with assessment notices, enabling assessment without cooperation. Best judgment assessment is required when a taxpayer fails to file a return (original, belated, or revised) or fails to comply with all terms of an inquiry notice that may require filing the return, producing accounts or documents, and furnishing verified information including assets and liabilities. It also applies where the taxpayer does not comply with directions for a special audit-entailing audit by a chartered accountant and inventory valuation by a cost accountant with a prescribed report-or where the taxpayer fails to comply with all terms of a scrutiny assessment notice.