Permanent Account Number requirement: PAN and Aadhaar linking mandated for eligible persons with prescribed authentication and quoting obligations. Persons meeting specified income, turnover, return-filing or transaction thresholds must apply for and be allotted a Permanent Account Number (PAN); PAN must be quoted in returns, correspondence and payment challans and changes to particulars must be notified. Aadhaar-eligible persons must quote Aadhaar on PAN applications and returns and intimate Aadhaar to the prescribed authority; failure may render PAN inoperative. Aadhaar may be used in lieu of PAN where PAN is not allotted or where both exist and Aadhaar has been intimated. The provision prohibits multiple PANs and mandates authentication of PAN/Aadhaar for prescribed transactions, with rulemaking and notification powers for prescribed classes and exemptions.