48 CFR §13.302-3

1. (a)When it is desired to consummate a binding contract between the parties before the contractor undertakes performance, the contracting officer shall require written (see 2.101) acceptance of the purchase order by the contractor.
2. (b)Each purchase order modification shall identify the order it modifies and shall contain an appropriate modification number.
3. (c)A contractor's written acceptance of a purchase order modification may be required only if—
   1. (1)Determined by the contracting officer to be necessary to ensure the contractor's compliance with the purchase order as revised; or
   2. (2)Required by agency regulations.