Tax audit requirement applies to businesses and professionals exceeding turnover or claiming lower presumptive profits, mandating accounts audit. Tax audit is mandatory for persons carrying on business whose turnover or gross receipts exceed the prescribed turnover threshold (with a higher threshold where cash transactions are minimal), and for professionals whose gross receipts exceed the professional threshold. Audit is also required where an eligible taxpayer claims profits lower than those computed under a presumptive taxation scheme, and opting out of a presumptive scheme triggers a multi-year bar on reverting to that scheme.