Australian Company Number

An Australian Company Number (usually shortened to ACN) is a unique nine-digit number issued by the Australian Securities and Investments Commission (ASIC) to every company registered under the Commonwealth Corporations Act 2001 as an identifier. The number is usually printed in three groups of three digits. It must be quoted on all correspondence and invoices issued by that company. A similar system is used for non-company entities such as trusts, and for foreign companies.
See more at Wikipedia.org...