Held that as the proceedings before the Principal Commissioner under section 263 of the Act were in the nature of collateral proceedings, the assessee could in the course of appellate proceedings originating from the order passed under section 263 of the Act, dated March 28, 2021 challenge the validity of the assessment order passed by the Assessing Officer under section 143(3), dated November 29, 2017. That when the order of reassessment under section 143(3), dated November 29, 2017 itself had been passed on the basis of invalid assumption of jurisdiction by the Assessing Officer and thus was invalid and bereft of any force of law or in fact non est in the eyes of law, it could not have been revised by the Principal Commissioner under section 263 of the Act. Assessment by Officer having jurisdiction pursuant to notice issued by  Officer not having jurisdiction-Non est.  Assessee within prescribed time objecting to jurisdiction of  Officer issuing notice. Entitled to challenge validity of  assessment on ground of  lack of jurisdiction.   (AY.2015-16)