20 C.F.R. § 1002.261
With the exception of multiemployer plans, which have separate rules discussed below, the employer is liable to the pension benefit plan to fund any obligation of the plan to provide benefits that are attributable to the employee's period of service. In the case of a defined contribution plan, once the employee is reemployed, the employer must allocate the amount of its make-up contribution for the employee, if any; his or her make-up employee contributions, if any; and his or her elective deferrals, if any; in the same manner and to the same extent that it allocates the amounts for other employees during the period of service. In the case of a defined benefit plan, the employee's accrued benefit will be increased for the period of service once he or she is reemployed and, if applicable, has repaid any amounts previously paid to him or her from the plan and made any employee contributions that may be required to be made under the plan.
20 C.F.R. § 1002.261