One theory is that it is possible to characterize the letter of credit as a guarantee agreement for a third party beneficiary, since the letters of credit are motivated by the buyer`s need and, in application of Jean Domat`s theory, the cause of a letter of credit is that a bank issues a loan in favor of a seller in order to release the buyer from his obligation to pay directly to the legal tender seller. In fact, there are three different entities that participate in the letter of credit transaction: the seller, the buyer, and the banker. Therefore, a letter of credit theoretically corresponds to a collateral contract accepted by the behavior, or in other words, to an implied contract. [8] In short, it is called LOC In a bilateral security contract, the two parties who enter into the main contract also enter into the security contract. A tripartite guarantee contract contains a bond issued by a third party that is not a party to the original contract. For example, if X agrees to purchase from Y goods that are manufactured accordingly by Z and does so on the basis of Z`s assurance of the high quality of the goods, X and Z may be concluded as having entered into a parallel contract consisting of Z`s quality promise, taking into account X`s quality promise, to conclude the main contract with Y. Parol proof rule, which means that the admissible proof of a parallel contract can be used to exclude the application of the Parol proof rule. In practice, it is rare to find a collateral contract as an exception, as it must be strictly proven; and the burden of proof is lightened only if the subject matter dealt with in the main contract is more unusual. [12] This rule prevents the parties from changing the meaning of written contracts by means of verbal or implied agreements not contained in the original contract, which results in this. .