Translate the following passage into Chinese. An L/C is a promised written legal document that is issued by the opening bank which promises to pay the holder if the holder fulfills certain obligations. Obligations include payment shall be effected when the goods are shipped if certain criteria are met. A letter of credit is usually used when the buyer and seller do not know each other very well. And this is why it is used so frequently in international trade. Letters of credit are incredibly specific and close intention to details is required.