WHERE CAN I FIND THIS IN THE FAR OR ANY GOVERNMENT ACQUISITION DOCUMENT.
The GAO and Courts have all said that a "within scope" determination is made by the contracting officer on a case-by-case basis. There is no specific FAR guidance on the subject. Here's some legal guidance:
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1. The following legal analysis concerning contract scope determinations, taken from the Government Contract Law, The Deskbook for Procurement Professionals, Third Edition, 2007 (pages 358 and 359), is applicable to this response. This publication is issued by the American Bar Association’s Section of Public Contract Law, but the text was written by Federal Government employees and is therefore in the public domain.
a. The tests used by the GAO, the Boards of Contract Appeals and the courts is (1) whether the change so materially alters the contract that the field of competition for the contract as modified would be significantly different from that obtained for the original contract (scope of competition) [AT&T Communications, Inc. v Wiltel, Inc., 1 F.3d 1201, 1205 (Fed. Cir. 1993)] and (2) whether the contract as modified, “should be regarded as having been fairly and reasonably within the contemplation of the parties when the contract was entered into” [Freund v. United States, 260 U.S. 60 (1922)].
b. In Hughes Space and Communications Co. B-276040, 97-1 CPD ¶ 158, the following factors were considered in determining whether the modification was in-scope: (1) the extent of any changes in the type of work or the performance period, or difference in costs between the contract as awarded and as modified, (2) whether the agency had historically procured the services under a separate contract, and (3) whether potential offerors would have anticipated the modification.
c. In determining the materiality of a change, the most important factor to consider is the extent to which a product or service, as changed, differs from the requirements of the original contract [E.L. Hamm & Associates, Inc., ASBCA No. 43792, 94-2 BCA ¶ 26724]. Additionally, an agency’s pre-award statements that certain work was outside the scope of the contract can bind the agency if it later attempts to modify the contract to include the work [Octel Communications Corp. v. Gen. Servs. Admin., GSBCA No. 12975-P, 95-1 BCA ¶ 27,315].
d. Changes in Quantity. Increases and decreases in the quantity of major items or portions of the work are not “within the scope” of the contract [Valley Forge Flag Co., Inc., VABCA Nos. 4667, 5103, 97-2 BCA ¶ 29,256; Liebert Corp., B-232234.5, Apr 29, 1991, 91-1 CPD ¶ 413]. Generally, increases are new procurements, and decreases are partial terminations [Cf. Lucas Aul, Inc., ASBCA No. 37803, 91-1 BCA ¶ 23,609].