The invoice most of the times is the last step in the sales process. It is a commercial document serving as an accounting voucher (to verify price, tax etc.). In general the invoice behaves like the order, it just represents a different state in the sales process. The invoices requests the customer to pay the value of products and services that have been delivered as agreed on in the order or have to be prepaid according to the payment modalities of the quote. Again there are the two ways of creating an invoice, from an order or from scratch. If the order has for example multiple milestones (payment modalities of type “due with delivery”) they should be billed separately so multiple invoices can be created from this order. One for each milestone but also multiple for each payment modality. If we have a unit “hour” we can create a monthly invoice that is related to one order with one line item and one payment modality. This is the main reason why QOIs need to be different entities and not a single entity with a status “QOI” as it is implemented in commerce kickstart for example. But invoices also need extra fields like credit period and skonto (not BETA) (early payment discount)