Basically the credits are either (i) revocable or (ii) irrevocable.
All credits should, therefore, indicate whether they are revocable or irrevocable, but in the absence of any indication, the credit will be treated as irrevocable as per Article 6(c) of UCP 500. Under UCP 400 the position was just the opposite i.e., in the absence of any indication, the Credit was deemed to be revocable.
The inherent weakness of revocable credit is that it may be cancelled or modified without prior notice by the issuing banker to the beneficiary. However, the opening bank is responsible for any operation on the revocable credit effected prior to the receipt by the negotiating bank of any cancellation or modification advice. An irrevocable letter of credit does not suffer from such disadvantage and it is, therefore, widely accepted.