This rule prevents parties from changing the importance of written contracts with oral or tacit agreements that are not included in the original contract, the latter impairing their integrity. This means that, when a contract is available in writing, subsequent agreements that are not entered into in writing are not proven in a contractual dispute. There are, however, several exceptions to this rule. The Common Law recognizes the collateral contract as an exception to the Parol rule of evidence, which means that the evidence authorized for a companion contract can be used to exclude the application of the Parol rule of evidence. In practice, it is rare to regard the warranty contract as an exception, as it must be strictly proven; and the burden of proof will only be lightened if the purpose with which the main contract is entered into is more unusual.  A party to an existing contract may attempt to demonstrate the existence of a security contract if its right to the infringement fails because the statement on which it was based is not considered to be the duration of the principal contract. It was decided that the explanation must have been so successful.  In the event of a breach of a security contract, corrective action may be taken. One theory confirms that it is possible to characterize creditworthy letters as an auxiliary contract for a third-party recipient, since letters of credit are driven by the need of the buyer and, in accordation of Jean Domat`s theory, to the cause of a letter of credit, a bank issues a credit in favour of a seller in order to exempt the buyer from his obligation to pay directly to the seller with a legal offer. There are three different companies involved in the letter of credit transaction: the seller, the buyer and the banker. Therefore, an accreditation contract is theoretically understood as a guarantee contract, which is accepted by a behaviour or, in other words, as a tacit contract.
 It is briefly called the LOC security contract, if it is falsified between the same parties as the main contract, it must not object to the main contract. In other words, if the term was agreed before the formal contract was concluded (but was still in place and could not be executed before the end of the second term), the first term will remain eligible.  In essence, security contracts cannot contradict an element of the main contract or the rights that flow from it.  Accompanying contracts are orally independent, between two parties to a separate agreement or between one of the original parties and a third party.3 min. Through a two-part support contract, the two parties entering into the main contract also enter into the security contract. A tripartite support contract includes a debt statement of a third party that does not participate in the original contract. This. B is often used in the case of a sales contract. A support contract is a contract by which the contracting parties enter into or promise another contract. The two treaties are therefore linked and can be applied, even if they are not a constructive part of the original treaty.
 In JJ Savage and Sons Pty Ltd v. Blakney, a mere expression of opinion was not deemed sufficient to be kept as a promise. In Crown Melbourne Limited v Cosmopolitan Hotel (Vic) Pty Ltd, a statement from a landlord to the tenants considered when negotiating a lease agreement that they are “supported during the extension” would not bind the lessor to offer another five-year lease.  Main contracts and security contracts are active simultaneously and, in some cases, the provisions of these contracts may replace the provisions of the former.