29 C.F.R. § 1620.4
An employee is engaged in the production of goods for interstate or foreign commerce within the meaning of the FLSA even if the employees's work is not an actual and direct part of such production, so long as the employee is engaged in a process or occupation which is "closely related" and "directly essential" to it. This is true whether the employee is employed by the producer of the goods or by someone else who provides goods or services to the producer. Typical of employees covered under these principles are computer operators, bookkeepers, stenographers, clerks, accountants, and auditors and other office and whitecollar workers, and employees doing payroll, timekeeping, and time study work for the producer of goods; employees in the personnel, labor relations, employee benefits, safety and health, advertising, promotion, and public relations activities of the producing enterprise; work instructors for the producers; employees maintaining, servicing, repairing or improving the buildings, machinery, equipment, vehicles or other facilities used in the production of goods for commerce, and such custodial and productive employees as watchmen, guards, firemen, patrolmen, caretakers, stockroom workers and warehousemen; and transportation workers bringing supplies, materials, or equipment to the producer's premises, removing waste materials therefrom, or transporting materials or other goods, or performing such other transportation activities, as the needs of production may require. These examples are illustrative, rather than exhaustive, of the employees who are "engaged in the production of goods for commerce" by reason of performing activities closely related and directly essential to such production.
29 C.F.R. §1620.4