Industrially backward district criteria determine eligibility for tax deduction under section 80-IB based on index thresholds and special conditions. Designation rules for industrially backward districts set objective thresholds using the All India Gradation List: Category A includes districts with a Total Weighted Index Count of 250 or less, 'no industry' districts, inaccessible hill area districts with a Total Weighted Index Count of 500 or less, and districts with no railhead as on 1-4-1994 with a Total Weighted Index Count of 500 or less; district boundaries follow the 1991 Census. Category B covers districts with a Total Weighted Index Count greater than 250 and up to 500, excluding districts already notified as Category A.