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A cafeteria plan is a vehicle that allows employees to pay for certain benefits on a pre-tax basis. It also saves the employer money on payroll taxes (e.g. FICA). To offer benefits on a tax-favored basis through a cafeteria plan, there must be a formal plan document, and the plan must comply with §125 rules such as election irrevocability rules and nondiscrimination rules. Without a cafeteria plan in place, employees may not pay for benefits on a pre-tax basis.
A cafeteria plan itself is not subject to ERISA, but underlying benefits run through the cafeteria plan probably are. A cafeteria plan is required to follow §125 rules rather than ERISA rules.