举个例，如果您已经就所交付的货物向买主开具了100元的发票，可是由于货物质量的瑕疵，买主主张“货接受但必须削价10元”，如果您接受了这个主张，那您就得开具10元的credit memo（即实际业务中的所谓“红字发票”）。总之，与原始invoice的价值相比，增价用debit memo，降价用credit memo，两者都是billing document type。

credit memo request 和debit memo request都是sales document type，其原理与一般的销售订单类型并无质的区别。只是，两个memo request的下游transaction都是直接参照订单创建billing document，而不需要有delivery。
credit memo request --> credit memo, debit memo request --> debit memo，这就是它们的流程。

Credit memo
The vendor has invoiced you too much for the last delivery, for example, less than the agreed quantity was delivered and that at the agreed total price, or you have returned part of a delivery to the vendor due to quality problems.
Subsequent debit
You have already received an invoice from your vendor for all the goods received. Subsequently, freight costs are to be taken into account, however, the invoice quantity remains the same.
Subsequent credit
You have already received a credit memo from your vendor for all the goods received. Subsequently, freight costs are to be credited to your company, however, the credit memo quantity remains the same.