48 CFR § 43.201 - General.
(a) Generally, Government contracts contain a changes clause that permits the contracting officer to make unilateral changes, in designated areas, within the general scope of the contract. These are accomplished by issuing written change orders on Standard Form 30, Amendment of Solicitation/Modification of Contract (SF 30), unless otherwise provided (see 43.301).
(b) The contractor must continue performance of the contract as changed, except that in cost-reimbursement or incrementally funded contracts the contractor is not obligated to continue performance or incur costs beyond the limits established in the Limitation of Cost or Limitation of Funds clause (see 32.706-2).
(c) The contracting officer may issue a change order by electronic means without a SF 30 under unusual or urgent circumstances, provided that the message contains substantially the information required by the SF 30 and immediate action is taken to issue the SF 30.