MAT adjustment for past year income under APA/secondary adjustment allows recomputation of past book profits and MAT relief. Finance Act, 2021 amended section 115JB to permit a company, by application, to have the assessing officer recompute book profit of past year(s) when current year book profit rises due to income included under an APA (section 92CC) or secondary adjustment (section 92CE); the AO will recompute tax, rectification time under section 154 runs four years from the end of the FY of application, relief is optional and excludes interest on refunds. Rule 10RB prescribes the MAT reduction formula (A-B)-(D-C), deeming zeros where no tax is payable, treats negative results as nil, reduces section 115JAA credit accordingly, and requires electronic claim in Form 3CEEA with specified verification and filing procedure.