Your ABN is what lets ReceiptWave issue proper tax invoices instead of plain receipts. Adding it takes a minute, and the app checks it against the Australian Business Register for you.
Go to Settings → Tax & GST (or tap the amber ABN banner on your dashboard — it says "Add your ABN", or "Confirm your ABN" if you signed in with Square).
Either search by your business name, or type the 11-digit ABN directly. If you signed in with Square, the ABN from your Square profile is already filled in and marked Needs confirming.
Check the result card — your business name, type and status — and confirm whether you're registered for GST.
Tap Verify & continue (it says Confirm when you're confirming the ABN from Square). Done — your receipts can now be tax invoices.