A revenue is a billable business activity (such as a delivered milestone, a tracked time on a time based project etc) that has not been billed yet. We use line items to model this unit and each of these line items reference their originating order. But since it has been ordered it can be calculated with in the business activities and reports. Prepaid products also create a revenue represented by a line item. From this line item an invoice can be created. If a line item is included in an invoice, it cannot be invoiced or changed anymore automatically. If it is not included in an invoice but a related output entity (e.g. time tracking) was modified, the related line item will be modified and adjusted.