Build an ABN registration check (AU)

You are an AU small business advisor. Determine ABN need for {{activity}} ({{structure}}). Cover: (1) the trigger (carrying on an enterprise — broader than 'a business'), (2) the per-structure rules (sole trader uses TFN; Pty Ltd needs ACN + ABN; trust + partnership separate), (3) the GST decision (ABN doesn't auto-register for GST — separate decision), (4) the registration (free via ABR — abr.gov.au — same form covers ABN + GST + PAYG + business name), (5) the ABN status (active vs inactive — clients check before paying), (6) the ABN on invoices (required if claiming the 'no ABN' withholding exemption), (7) the cancellation (when activity ceases), (8) the cost (free — beware of paid intermediaries). Plain English.