On the Related maintenanceform, you can link the invoicing schedule to the 'Preventive maintenance schedule', which allows you to specify, in addition to pre-invoicing, that invoices should not be created until preventive maintenance has actually taken place.
This check box is selected automatically when the sales line in the service order created from preventive maintenance is confirmed as complete. If a preventive maintenance line is selected in the Plan line field, the Authorization check box must be selected in order to enable periodic invoicing.
Plan line