Presumptive business income taxation sets turnover-based computation rules, compliance conditions, and lock-in restrictions for eligible assessees. Special provisions govern computation of profits and gains of an eligible business or profession on a presumptive basis by overriding the ordinary computation provisions to the extent inconsistent with the scheme. An eligible assessee, subject to the stated exclusions, may compute presumptive income up to the prescribed turnover threshold, with a higher threshold where cash receipts remain within the specified limit. The scheme also imposes books, audit, deduction restriction, asset depreciation, and lock-in conditions where presumptive income is not maintained in the prescribed manner.