48 CFR §43.103

1. (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. (1)Make negotiated equitable adjustments resulting from the issuance of a change order;
   2. (2)Definitize letter contracts; and
   3. (3)Reflect other agreements of the parties modifying the terms of contracts.
2. (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. (1)Make administrative changes;
   2. (2)Issue change orders;
   3. (3)Make changes authorized by clauses other than a changes clause (e.g., Property clause, Options clause, or Suspension of Work clause); and
   4. (4)Issue termination notices.