Registration requirement for trusts conditions tax exemption and mandates audit and return compliance for applicability of exemption provisions. Section 12A makes registration under section 12AA or 12AB a prerequisite for sections 11 and 12 to apply to a trust or institution's income; applications must be made in prescribed form to the Principal Commissioner or Commissioner within specified timeframes tied to creation, commencement, expiry or modification of objects, with discretion to condone delay. Where total income (without applying sections 11 and 12) exceeds the non-taxable threshold, prescribed books must be maintained, accounts audited by a defined accountant, and the audit report and statutory returns furnished by specified dates.