Irrovocable letter of credit是什么?

Irrovocable letter of credit是什么? DISCREPANCY - LETTER OF CREDIT 信用证Letter of Credit letter of credit是什么意思 commercial letter of credit L/C=letter of credit,信用证是什么? confirmed letter of credit是什么意思 Revolving Letter of Credit是? revolving documentary letter of credit documentary letter of credit是什么意思 open letter of credit是什么意思 INVOICE,PACKING LIST,BILL OF LADING,LETTER OF CREDIT这些词汇是什么意