A Business Number relates to a business as a Social Insurance Number (SIN) relates to an individual. This is not a business registration. The Canada Revenue Agency (CRA), issues this nine-digit number when you register for any the of 4 CRA program accounts:
HST/GST
payroll deductions (Income tax, Canada Pension Plan, Employment Insurance)
importer/exporter license
corporate income tax number
This number is designed to simplify and streamline interaction with CRA. Related information is available at the Canada Revenue Agency website.
FYI: A Business Number (BN) is not to be confused with a Business Identification Number (BIN). If you register your business name, you will receive a Business Identification Number as part of your Business Name Registration from the Ontario Ministry of Consumer and Business Services. The BN number is issued by the Federal Government.