If a credit requires expressly a master air waybill, why a house air waybill is unacceptable, although ICC Document 470/TA.621 says it is acceptable?

• 如信用状明示要求母空运单，为何不能接受子空运单，尽管国际商会银委会第470/TA.；621号文件认为此亦可以接受？