5 C.F.R. § 316.904
Post-secondary student positions under the General Schedule or appropriate pay plan must be classified to the -99 series of the appropriate occupational group. Federal Wage System positions filled under the authority in this subpart must be classified to the -01 series of the appropriate occupational group. Agencies may refer to OPM's, "Introduction to the Position Classification Standards" at https://www.opm.gov/policy-data-oversight/classification-qualifications/classifying-general-schedule-positions/positionclassificationintro.pdf for a definition of these positions. In addition, agencies can refer to the "Handbook of Occupational Groups and Families" available at https://www.opm.gov/policy-data-oversight/classification-qualifications/classifying-general-schedule-positions/occupationalhandbook.pdf.
5 C.F.R. §316.904