20 CFR 404.1053: “Qualified benefits” under a cafeteria plan.

We do not include as wages any qualified benefits under a cafeteria plan as described in section 125 of the Code if such payment would not be treated as wages without regard to such plan and it is reasonable to believe that (if section 125 applied for purposes of this section) section 125 would not treat any wages as constructively received. This includes any qualified benefit made to you, or on your behalf, pursuant to a salary reduction agreement between you and your employer. The Internal Revenue Service decides whether any plan is a cafeteria plan under section 125 of the Code and whether any benefit under the plan is a qualified benefit.