In Australia, an ACN (Australian Company Number) is a unique 9-digit number issued by ASIC to companies registered under the Corporations Act. It identifies a company as a separate legal entity and must be displayed on company documents and public records.
An ABN (Australian Business Number) is an 11-digit number issued by the Australian Business Register to identify businesses when dealing with the government, suppliers, and customers. Sole traders, partnerships, trusts, and companies can all have an ABN, and companies usually hold both an ABN and an ACN.