Trip invoice lines are sales invoice lines, directly entered in a trip (tab sheet Invoice lines). This can be generated automatically when an order is dragged unto a trip from another company. Or through a custom action.
Trip invoice lines have a debtor. And are collect on a sales invoice. In the screen sales invoices these lines are collected on an invoice as separate lines.
By using trip invoice lines you can send a bill for a trip, without the need to place bookings within a trip.