Treas. Reg. § 1.431(c)(6)-1

(a) Mortality tables used to determine current liability. The mortality assumptions that apply to a defined benefit plan for the plan year pursuant to section 430(h)(3)(A) and § 1.430(h)(3)-1(a) are used to determine a multiemployer plan's current liability for purposes of applying the rules of section 431(c)(6). Either the generational mortality tables used pursuant to § 1.430(h)(3)-1(a)(2) or the static mortality tables used pursuant to § 1.430(h)(3)-1(a)(3) may be used for a multiemployer plan for this purpose. However, for this purpose, substitute mortality tables under § 1.430(h)(3)-2 may not be used for a multiemployer plan.

(b) Effective/applicability date. This section applies for plan years beginning on or after January 1, 2018. For rules that apply to plan years beginning before January 1, 2018 and on or after January 1, 2008, see § 1.431(c)(6)-1 (as contained in 26 CFR part 1 revised April 1, 2015).