There is a general rule of thumb to apply when distinguishing between subcontractors and consultants. A subcontractor is hired to perform a part of the prime contractor's statement of work (SOW). Such work might include subsystems, finished parts, or skilled labor to perform specific functions. For example, a base maintenance contractor might subcontract barracks painting requirements to a painting company. Or, an aircraft manufacturer might subcontract the avionics portion of a contract to a company that specializes in avionics.
Consultants are not usually hired for a specific contract or to complete a portion of the prime contractor's SOW. They are often subject matter experts whose expertise is not tied to a single contract.
FAR Definitions. The Federal Acquisition Regulations (FAR) have a few definitions that will help clarify the difference between subcontractors and consultants.
FAR 3.502-1: Subcontractor (1) means any person, other than the prime contractor, who offers to furnish or furnishes any supplies, materials, equipment or services of any kind under a prime contract or a subcontract entered into in connection with such prime contract and (2) includes any person who offers to furnish or furnishes general supplies to the prime contract or a higher tier subcontractor.
FAR 3.1001: Subcontractor means any supplier, distributor, vendor, or firm that furnished supplies or services to or for a prime contractor or another subcontractor.
FAR 31.205-33: Professional and consultant services ... means those services rendered by persons who are members of a particular profession or possess s special skill and who are not officers or employees of the contractor. Examples include those services acquired by a contractor or subcontractor in order to enhance their legal, economic, financial, or technical positions. Professional and consultant services are generally acquired to obtain information, advice, opinions, alternatives, conclusions recommendations, training or direct assistance, such as studies, analyses, evaluations, liaison with Government officials, or other forms or representation.