Claiming a lower DTAA withholding rate in India -- say, 15% instead of the domestic 31.2% on NRO interest -- generally requires a Tax Residency Certificate from the country you actually live in. For someone tax-resident in the US, that means requesting Form 6166, the IRS's official Certificate of U.S. Tax Residency, and the way to request it is Form 8802, Application for United States Residency Certification.
How the request works
Form 8802 is filed with the IRS along with the required user fee; the IRS then issues Form 6166, a letter on Treasury stationery certifying US tax residency for the years requested. Processing typically takes 30-45 days, and can stretch toward 60 days during the January-April filing season, so the IRS's own guidance is to apply at least 45 days before the certificate is actually needed by an Indian bank, tenant, or buyer.
Form 6166 by itself is not the whole story on the Indian side: Form 10F is the self-declaration NRIs additionally file to give Indian tax authorities the specific details -- taxpayer identification number, address, period of residency -- that the TRC alone doesn't spell out. Skip either piece and the payer defaults to the higher domestic withholding rate, refundable only by filing an Indian return.
Practical timing
Because Form 6166 is typically requested per calendar year, and processing isn't instant, this isn't a form to think about only when a bank suddenly asks for it. Filing Form 8802 early in the year -- before an NRO fixed deposit renews or a property sale is expected to close -- avoids paying the higher rate up front and then having to chase a refund through an Indian tax return later.