48 CFR 43.103: 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.