(810 ILCS 5/2A-202) (from Ch. 26, par. 2A-202)

Sec. 2A-202.

Final written expression; parol or
extrinsic evidence.
Terms with respect to which the confirmatory memoranda of
the parties agree or which are otherwise set forth in a writing
intended by the parties as a final expression of their agreement
with respect to such terms as are included therein may not be
contradicted by evidence of any prior agreement or of a
contemporaneous oral agreement but may be explained or
supplemented:

(a) by course of dealing or usage of trade or by course of
performance; and

(b) by evidence of consistent additional terms unless the court finds the writing to