48 C.F.R. § 816.203-4
(e) The contracting officer shall, when contracting by negotiation, use the following clauses.
(1) The contracting officer shall insert the clause at 852.216-71, “Economic Price Adjustment of Contract Price(s) Based on a Price Index,” in solicitations and firm fixed price contracts, subject to FAR 16.203-4(d)(1) and when changes to a price index will be used to calculate corresponding changes to the total contract price or unit prices of the contract.
(i) Exceptions:
(2) The contracting officer shall insert the clause at 852.216-72, “Proportional Economic Price Adjustment of Contract Price(s) Based on a Price Index,” in solicitations and firm fixed price contracts, and subject to FAR 16.203-4(d)(1) when changes to an industry price index shall be used to calculate changes to only a portion of the contract price or the unit prices of the contract.
(i) Exceptions: