Advance tax default rules: failure to pay scheduled instalments or to file required estimates triggers deeming as assessee in default. Deeming provisions treat a taxpayer as an assessee in default when instalments of advance tax are not paid as per the prescribed schedule after submission of a statement, estimate, or revised estimate, or when instalments are due without a prior estimate; valid statutory deferments exclude specified amounts from default until the deferred payment date communicated to the tax authority.