1，如果这个供应商为invoice match option-receipt. 那么默认的在ap account做invoice match的时候， 会跟receipt作为match.
             同样的选择po,就会默认和po match。

一般po shipment曾有三种，2-way, 3-way, 4-way
2-way的情况，只要你在po approval后，就可以建立invoice,这时候match po，invoice validation规则为po order qty和invoice qty比较
3-way的情况，是在shipment作完reeipt后，建立invoice，match receipt，validation多receipt qty

2-way matching
invoice price <= order price
quantity billed<= quantity ordered

3-way matching (standard)
invoice price <= order price
quantity billed<= quantity ordered
quantity billed<= quantity received

4-way matching    (require inspection)
invoice price <= order price
quantity billed<= quantity ordered
quantity billed<= quantity received
quantity billed<= quantity accepted