48 CFR §16.301-2

1. (a)The contracting officer shall use cost-reimbursement contracts only when—
   1. (1)Circumstances do not allow the agency to define its requirements sufficiently to allow for a fixed-price type contract (see 7.105); or
   2. (2)Uncertainties involved in contract performance do not permit costs to be estimated with sufficient accuracy to use any type of fixed-price contract.
2. (b)The contracting officer shall document the rationale for selecting the contract type in the written acquisition plan and ensure that the plan is approved and signed at least one level above the contracting officer (see 7.103(j) and 7.105). See also 16.103(d).