FEI number stands for Facility Establishment Identification number, which is a unique identifier issued by the FDA to track inspections of the regulated establishment or firm. It is a nine-digit number assigned by the Internal Revenue Service to business entities operating in the United States for the purposes of identification. The FEI number is used to identify a firm and is generated by the FDA system. It is used for tax administration and must not be used for any other purpose. The FEI number is also used to track GDUFA (Generic Drug User Fee Amendments) fees. The FEI number is not mandatory for all drug facilities to be registered with the FDA, but it is necessary for some facilities such as human generic drug and active pharmaceutical ingredient manufacturers, finished dosage manufacturers, facilities involved in packaging and labeling of drugs, and bioequivalence study centers.