7 C.F.R. § 3402.8
A USDA Graduate Fellow shall be enrolled as a full-time graduate student, as defined by the institution, at all times during the tenure of the Fellowship in the national need area and at the degree level supported by the grant. This includes the time used for special international study or thesis/dissertation research, if the international travel is funded through a special international study or thesis/dissertation research travel allowance under this grant program. However, the normal requirement for formal registration during part of this tenure may be waived if permitted by the policy of the Fellowship institution, provided that the Graduate Fellow is making satisfactory progress toward degree completion and remains engaged in appropriate full-time Fellowship activities such as thesis/dissertation research. Postdoctoral Fellowship appointments may be held only by persons who pursue full-time traineeship in research, teaching, or extension in the national need area and are supervised by the mentor indicated in the grant application. Graduate and postdoctoral Fellows in academic institutions are not entitled to vacations as such. They are entitled to the short normal student holidays observed by the institution. The time between academic semesters or quarters is to be utilized as an active part of the grant period. During the period of support, USDA Graduate and Postdoctoral Fellows are permitted, at the discretion of their institutions, to accept additional supplemental employment that would positively contribute to their training or research and provide eligibility for tuition waivers (e.g., full or partial tuition waivers provided with research or teaching assignments). A Fellow may accept from any other entity a grant supporting the Fellow's research costs.
7 C.F.R. § 3402.8