Line items for opportunities whose invoice type is Fixed Price Open Billing behave differently. They are not preceded by a checkbox.
Once a work order of invoice type Fixed Price Open Billing has been won, it becomes available in the Invoicing Assistant.
As incremental invoices are submitted for the work being completed, the invoice recommendation will remain in the Invoicing Assistant until the work order has been fully invoiced.
Clicking on a line item that is Ready or Overdue, displays the Invoice Open Billing screen.