Estimates, invoices and change orders are numbered the same way: a prefix you set in Settings, a counter that steps up by one, padded to at least three digits — EST-001, INV-042, CO-007.
The counter advances the first time a document is saved, when an estimate is duplicated or converted, when a recurring invoice generates, and when a client signs an estimate in their portal. Prefixes and next numbers are all editable in Settings.