20 C.F.R. § 323.2
A nongovernmental plan for unemployment or sickness insurance is a benefit plan, program or policy that is in the nature of insurance and is designed and established by an employer for the purpose of supplementing the benefits that an employee of such employer may receive under the Railroad Unemployment Insurance Act during a period of unemployment or sickness. A nongovernmental plan may be established by labor-management agreement or by unilateral employer action. Payments under such plans are referred to as supplemental unemployment benefits (SUB pay) or supplemental sickness benefits, rather than as wages, salary or pay for time lost, because their inherent nature is to supplement benefit payments under the Railroad Unemployment Insurance Act rather than to replace or duplicate such payments.
20 C.F.R. § 323.2