求翻译：凭保兑、不可撤销的即期信用证凭单付款。信用证以卖方为受益人，该有效信用证必须在装运期15天前开到卖方，信用证有效期不少于在装船后的21天，否则卖方无需通知即可有权取消本合同, 并向买方索赔因此而发生的相应损失。是什么意思？

With confirmation, irrevocable sight letter of credit voucher payment. Letter of credit in favor of the seller, the letter of credit must be effective 15 days before shipment to the vendor to open letters of credit valid for not less than 21 days after shipment, or without notice to the seller the r

confirmer by virtue, irrevocable letters of credit at sight payment vouchers. Letters of credit for the seller as the beneficiary, the valid period for credit must be in 15 days before shipment to the seller, the letter of credit is valid for not less than 21 days after the ship without notice, othe

Depends on immediate letter of credit certificate payment which guarant

By confirmed, irrevocable letter of credit vouchers for payment. The seller beneficiary of letter of credit, the valid letter of credit must be open to the seller before shipment by 15 days, letter of credit is valid for not less than 21 days after shipment, otherwise the seller can be right to canc