Xero bills can have the status "Draft" and "Approved".
By default bills submitted to Xero will have the "Draft" status. To export a bill with the "Approved" status you can tick the "Approved" checkbox in the BillBjorn review screen.
To mark all incoming bills as "Approved" you can tick the "Mark Documents As Approved" checkbox under
Company Settings > Preferences > Mark Documents As Approved
This setting is only available for the Xero integration.