Registration under section 12AA protects trusts' tax exemption for pending prior-year income and limits reassessment powers. Where registration under section 12AA is granted, sections 11 and 12 apply to income from property for any preceding assessment year with pending assessment proceedings as on the date of registration, provided the trust's objects and activities remained the same for those years; the Assessing Officer shall not take action under section 147 for such preceding years solely for non-registration, but these provisos do not apply if registration was refused or later cancelled.