agreement is not admissible in most instances. See David Nassif Assoc. v. United States, 214 Ct.Cl. 407, 557 F.2d 249, 256 (1977). The parol evidence rule is a rule of substantive law that “prohibits ... the use of external evidence to add to or otherwise modify the terms of a written agreement in instances where the written agreement has been adopted by the parties as an expression of their final understanding.” David Nassif Assoc., 214 Ct.Cl. at 419-420, 557 F.2d 249. Such an agreement is often called an integrated agreement. See Barron Bancshares, 366 F.3d at 1376. If an integrated agreement is “clear and unambiguous, we must construe [it] according to [its] plain meaning, without resort to parol evidence.” Id.; see also Short Bros., PLC v. United States, 65 Fed.Cl. 695, 781 (2005) (<HOLDING>). This rule, at its heart, is aimed at
holding that extrinsic evidence of the parties course of conduct may be considered where the contract language is ambiguous holding that where an instrument is ambiguous all relevant extrinsic evidence may properly be considered in resolving the ambiguity holding that court may not use extrinsic evidence unless contract language is ambiguous holding that extrinsic evidence is permissible to interpret an ambiguous contract holding that a court may consider extrinsic evidence to determine whether a contract is ambiguous