Drupal Commerce differentiates between the serial numeric ID assigned to an order by the database on save and the order number used to formally identify a placed order. Orders typically start as "drafts" (as in the case of shopping carts) and are assigned an order number when placed (as when a customer completes checkout).
In Commerce 1.x, the order number defaulted to the order ID. This created confusing order number sequences in admin listings when carts were checked out long after they were created or when they were abandoned. Businesses that required sequential order numbers without gaps needed custom code to make it happen.
Thanks to the Number Pattern module we added in Commerce 2.x, site builders have much greater control over order number generation via configuration alone. Number Pattern is used to generate order numbers and invoice numbers (via Commerce Invoice), and it is available for other contributed modules to use as well.
See selection here in the order type edit form:
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