48 C.F.R. § 31.102
The applicable subparts of part 31 shall be used in the pricing of fixed-price contracts, subcontracts, and modifications to contracts and subcontracts whenever (a) cost analysis is performed, or (b) a fixed-price contract clause requires the determination or negotiation of costs. However, application of cost principles to fixed-price contracts and subcontracts shall not be construed as a requirement to negotiate agreements on individual elements of cost in arriving at agreement on the total price. The final price accepted by the parties reflects agreement only on the total price. Further, notwithstanding the mandatory use of cost principles, the objective will continue to be to negotiate prices that are fair and reasonable, cost and other factors considered.
48 C.F.R. § 31.102