The contracting officer, often shortened to CO or KO, is the only person on a federal deal with the legal authority to sign a contract, change one, or commit the government to spend money. Everyone else you talk to on a procurement, a program manager, a technical evaluator, a contract specialist, can advise and negotiate, but cannot bind the government on their own. If someone other than the CO tells you to start work or change scope, get it confirmed by the CO in writing before you act on it.

In a sentence

"All contract actions must be signed by the contracting officer or an authorized representative." A friendly conversation with a program manager is not a contract change; only the CO's signature, or written direction from someone the CO has formally delegated, makes anything official.