48 CFR § 43.103 - Types of contract modifications.
Contract modifications are of the following types:
(a) Bilateral. A bilateral modification (supplemental agreement) is a contract modification that is signed by the contractor and the contracting officer. Bilateral modifications are used to—
(1) Make negotiated equitable adjustments resulting from the issuance of a change order;
(2) Definitize letter contracts; and
(3) Reflect other agreements of the parties modifying the terms of contracts.
(b) Unilateral. A unilateral modification is a contract modification that is signed only by the contracting officer. Unilateral modifications are used, for example, to—
(1) Make administrative changes;
(2) Issue change orders;
(3) Make changes authorized by clauses other than a changes clause (e.g., Property clause, Options clause, or Suspension of Work clause); and
(4) Issue termination notices.