A GIIN is a Global Intermediary Identification Number assigned to Foreign Financial Institutions (FFI), Financial Institution (FI) branches, direct reporting non-financial foreign entities (NFFE), sponsoring entities, sponsored entities, and sponsored subsidiary branches. An Institution or Entity assigned a GIIN can use it to identify themselves to withholding agents and tax administrators for FATCA reporting purposes. This, however, is not a TIN in the USA. A Tax Identification Number (TIN) for the USA is an identification number used by the Internal Revenue Service (IRS) in the administration of tax laws. It is issued either by the Social Security Administration (SSA) or by the IRS. The US is the only jurisdiction that issues GIINs for FATCA purposes but other jurisdictions issue TINs for tax purposes. The OECD has issued overview of TINs issued by member jurisdictions https://www.oecd.org/tax/automatic-exchange/crs-implementation-and-assistance/tax-identification-numbers/