 The rule is generally binding only on the parties to the contract, not on third persons, for the latter may normally lead evidence to contradict or vary the contents of the contract.When, however, the issue in dispute (even between third parties) is what the obligations of the contracting parties to one another are, and those obligations are stated in a written contract, the integration rule is applicable.It must be noted that the rule does not apply to oral agreements made after the written document was completed.