NRI I-T filing vs Tax payers Identification number

Sir. I am filing NRI income tax return, but the system is asking the "Tax payers Identification Number". The assessee is at USA, and he says though he is filing income tax returns in USA, But he does not posses the " Tax Identification Number" but he has only Social Security Number , so now my doubt is they both are one at the same or different.? kindly advice.