Tax residency certification: prescribed Forms 10FA/10FB and verification requirements for treaty relief take effect, defining documentary and AO procedures. Rule 21AB prescribes the form and content of certificates of residence for claiming relief under sections 90 and 90A: foreign governments must verify and provide specified particulars (name, status, nationality/incorporation, tax identification or unique identifier, tax residential status, applicable period and address). Indian residents must apply to the Assessing Officer in Form No. 10FA, who may issue a residence certificate in Form No. 10FB containing name, status, PAN, address, validity period and the AO's identification and seal.