18 C.F.R. § 367.30
Service companies must maintain a detailed classification of service costs, that permits costs to be identified with the functional processes of the associate companies served. To permit the classification, each service company must maintain a cost accumulation system, as described in Definitions § 367.1(a)(12) , for accumulating reimbursable costs and charges to the associate companies served, and maintain time records for all service company employees in order to support the accounting allocation of all expenses assignable to the types of services performed and chargeable to the associate companies served. Service company employee records must permit a ready identification of the hours worked, account numbers charged, and other code designations that facilitate proper classification.
18 C.F.R. §367.30