What is a UBI (Unified Business Identifier) number?

A UBI number is a 9-digit number that registers you with several state agencies and allows you to do business in Washington State. A UBI number is sometimes called a tax registration number, a business registration number, or a business license number. Use the Business License Application to apply for a UBI number.

Source: http://bls.dor.wa.gov/faqlicense.aspx