(1) Purpose. When a multiemployer plan terminates by the withdrawal of every employer from the plan, or when substantially all employers withdraw from a multiemployer plan pursuant to an agreement or arrangement to withdraw from the plan, section 4219(c)(1)(D)(i) of ERISA requires that the liability of such withdrawing employers be determined (or redetermined) without regard to the 20-year limitation on annual payments established in section 4219(c)(1)(B) of ERISA. In addition, section 4219(c)(1)(D)(ii) requires that, upon the occurrence of a withdrawal described above, the total unfunded vested benefits of the plan be fully allocated among such withdrawing employers in a manner that is not inconsistent with PBGC regulations. Section 4209(c) of ERISA provides that the de minimis reduction established in sections 4209 (a) and (b) of ERISA does not apply to an employer that withdraws in a plan year in which substantially all employers withdraw from the plan, or to an employer that withdraws pursuant to an agreement to withdraw during a period of one or more plan years during which substantially all employers withdraw pursuant to an agreement or arrangement to withdraw. The purpose of subpart B of this part is to prescribe rules, pursuant to sections 4219(c)(1)(D) and 4209(c) of ERISA, for redetermining an employer's withdrawal liability and fully allocating the unfunded vested benefits of a multiemployer plan in either of two mass-withdrawal situations: the termination of a plan by the withdrawal of every employer and the withdrawal of substantially all employers pursuant to an agreement or arrangement to withdraw. Subpart B also prescribes rules for redetermining the liability of an employer without regard to section 4209 (a) or (b) when the employer withdraws in a plan year in which substantially all employers withdraw, regardless of the occurrence of a mass withdrawal. (See part 4281 regarding the valuation of unfunded vested benefits to be fully allocated under subpart B, and parts 4041A and 4281 regarding the powers and duties of the plan sponsor of a plan terminated by mass withdrawal.)

(2) Scope. Subpart B applies to multiemployer plans covered by title IV of ERISA, with respect to which there is a termination by the withdrawal of every employer (including a plan created by a partition pursuant to section 4233 of ERISA) or a withdrawal of substantially all employers in the plan pursuant to an agreement or arrangement to withdraw from the plan, and to employers that withdraw from such multiemployer plans. The obligations of a plan sponsor of a mass-withdrawal-terminated plan under subpart B cease to apply when the plan assets are distributed in full satisfaction of all nonforfeitable benefits under the plan. Subpart B also applies, to the extent appropriate, to multiemployer plans with respect to which there is a withdrawal of substantially all employers in a single plan year and to employers that withdraw from such plans in that plan year.