Here is the breakdown of possible statuses in Fortnox:
Order track:
Order created
Order updated
(Warehouse Ready / Delivered) — optional status on the order
Order converted → becomes an Invoice
Invoice track (after conversion):
Invoice created (draft, unbooked — ej bokförd)
Invoice booked (bokförd) — posted to ledger, legally binding
Invoice sent (optional — if Send Invoice is enabled)
Invoice paid / partially paid — when payment is registered against it
Invoice fully closed — zero outstanding balance
This setting, Book keep converted invoices, is automatically booking a newly created invoice. This is usually recommended in the Fortnox setup.