Block assessment procedure treats undisclosed income from searches as block period income to be assessed accordingly. A special assessment regime requires the Assessing Officer to treat total undisclosed income found in searches or requisitions as income of a ten year block period, computed under Chapter IV on the basis of evidence from searches, concluded assessments, filed returns or recorded book entries, with carry forward losses and unabsorbed depreciation not set off against block income. The assessee bears the burden of proving prior disclosure. Procedural rules prescribe a minimum period for filing a prescribed return, application of specified assessment provisions, retention and transfer of seized materials, a one year time limit for completion, exemption from certain interest and specified penalties, and minimum rank and prior approval requirements for the assessing authority.