Aadhaar PAN linking requirement: failure to intimate Aadhaar renders PAN inoperative, restricting refunds and increasing withholding obligations. Every person eligible for Aadhaar must quote Aadhaar number or Enrolment ID when applying for PAN or filing returns under section 139AA(1). Persons holding PAN as of the statutory trigger must intimate Aadhaar under section 139AA(2) and Rule 114AAA; failure to do so leads to the PAN being made inoperative with consequences including withholding of refunds, non-payment of interest on refunds for the prescribed period, higher tax deduction or collection rates, and payment of a fee to restore operativity. Exemptions may be notified by the Central Government.