A Global Intermediary Identification Number (GIIN) is a unique identification number issued by the Internal Revenue Service (IRS) or the relevant tax authority of the jurisdiction where the intermediary is located. It is used to identify financial institutions and other intermediaries that are required to report under FATCA. Part III of Form 8966 is used to report the GIIN of the reporting financial institution or withholding agent. If the filer is an FFI (Foreign Financial Institution) or NFFE (Non-Financial Foreign Entity) reporting directly on Form 8966, they will need to apply for and obtain their own GIIN.