29 C.F.R. § 4.153
There may be employees used by a contractor or subcontractor in performing a service contract in excess of $2,500 which is subject to the Act, whose services, although necessary to the performance of the contract, are not subject to minimum monetary wage or fringe benefit provisions contained in the contract pursuant to section 2(a) because such employees are not directly engaged in performing the specified contract services. An example might be a laundry contractor's billing clerk performing billing work with respect to the items laundered. In all such situations, the employees who are necessary to the performance of the contract but not directly engaged in the performance of the specified contract services, are nevertheless subject to the minimum wage provision of section 2(b) (see § 4.150 ) requiring payment of not less than the minimum wage specified under section 6(a)(1) of the Fair Labor Standards Act to all employees working on a covered contract, unless specifically exempt. However, in situations where minimum monetary wages and fringe benefits for a particular class or classes of service employees actually performing the services called for by the contract have not been specified in the contract because the wage and fringe benefit determination applicable to the contract has been made only for other classes of service employees who will perform the contract work, the employer will be required to pay the monetary wages and fringe benefits which may be specified for such classes of employees pursuant to the conformance procedures provided in § 4.6(b) .
29 C.F.R. §4.153