Federal Acquisition Regulation (FAR) 1.602-2 says that, "Contracting officers are responsible for ensuring performance of all necessary actions for effective contracting..." FAR 1.602-2(d) it says that that contracting officers shall, "Designate and authorize, in writing, a contracting officer's representative (COR) on all contracts and orders other than those that are firm-fixed price, and for firmed-fixed price contracts and orders as appropriate. However, the contracting officer is not precluded from retaining and executing the COR duties as appropriate." Defense Federal Acquisition Regulation Supplement (DFARS) 201.602-70 says, "Use of the clause at DFARS 252.201-7000, Contracting Officer's Representative, in solicitations and contracts when appointment of a contracting officer's representative is anticipated."
Department of Defense policy says that the requiring activity responsible for technical requirements also prescribes contract quality requirements and nominates COR's to assist in the technical monitoring or administration of contracts. Ideally when a COR is required the requiring activity will provide the COR nomination to the contracting office as part of the purchase request, see Deputy Secretary of Defense Memorandum, "Monitoring Contract Performance in Contracts for Services." August 22, 2008.
Agencies may have further guidance on when to appoint COR's. Generally, the contracting officer is responsible for ensuring satisfactory performance and timely deliver under the contract and a COR is the technical liaison between the contractor and the contracting officer appointed in writing to assist when needed.