什么叫 an unopen letter of credit 如何定义的?

什么叫 an unopen letter of credit 如何定义的? letter of credit 前用什么不定冠词?an letter of credit或者an L/C,为何不是a DISCREPANCY - LETTER OF CREDIT 信用证Letter of Credit letter of credit是什么意思 commercial letter of credit an irrevocable and workable letter of credit payable at sigh the buyer shall establish an irrevocable letter of credit at confirmed letter of credit是什么意思 Revolving Letter of Credit是? Irrovocable letter of credit是什么? revolving documentary letter of credit