29 CFR 4219.2 - Definitions.
(b) For purposes of this part:
Mass withdrawal valuation date means -
(2) in the case of a withdrawal of substantially all employers pursuant to an agreement or arrangement to withdraw, the last day of the plan year as of which substantially all employers have withdrawn. Nonforfeitable benefit means a benefit described in § 4001.2 of this chapter plus, for purposes of this part, any adjustable benefit that has been reduced by the plan sponsor pursuant to section 305(e)(8) of ERISA and section 432(e)(8) of the Code that would otherwise have been includable as a nonforfeitable benefit.
Reallocation record date means a date selected by the plan sponsor, which shall be not earlier than the date of the plan's actuarial report for the year of the mass withdrawal and not later than one year after the mass withdrawal valuation date.
Unfunded vested benefits means the amount by which the present value of a plan's vested nonforfeitable benefits (as defined for purposes of this section) exceeds the value of plan assets (including claims of the plan for unpaid initial withdrawal liability and redetermination liability), determined in accordance with section 4281 of ERISA and part 4281, subpart B.
(c) For purposes of subpart B -