FEIN stands for Federal Employer Identification Number. It is a unique nine-digit number assigned by the Internal Revenue Service (IRS) to business entities operating in the United States for the purpose of identification. The FEIN is used by employers for the filing of employment tax returns and other tax forms. SINN is an acronym for Social Insurance Number, which is a nine-digit number issued by the Canadian government to individuals for the purpose of tracking their income and benefits.