A “merger clause” is a common provision in a contract. It will usually state something along the lines of the contract being a “complete and exclusive statement of the agreement.” If a contract is a final, complete agreement, the parol evidence rule prohibits prior or contemporaneous oral agreements which vary or contradict the written terms (unless there if fraud, mistake, or accident). CIT Group/Sales Financing, Inc. v. Lark, 906 S.W.2d 865, 868 (Mo. App. E.D. 1995). In other words, you can only look at the document itself when interpreting the agreement.
The presence of a merger clause suggests that the contract is a final writing. Specifically, it is given “great weight and aids the court in determining whether the writing is the final expression of all terms agreed upon.” Johnson v. JF Enterprises, LLC, 400 S.W. 3d 763, 766 (Mo. 2013).