Treas. Reg. § 1.433(h)(3)-1

(a) Mortality tables used to determine current liability. In accordance with section 433(h)(3)(B), 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 CSEC plan's current liability for purposes of applying the rules of section 433(c)(7)(C). Either the static mortality tables used pursuant to § 1.430(h)(3)-1(a)(3) or generational mortality tables used pursuant to § 1.430(h)(3)-1(a)(2) may be used for a CSEC plan for this purpose, but substitute mortality tables under § 1.430(h)(3)-2 may not be used for this purpose.

(b) Effective/applicability date. This section applies for plan years beginning on or after January 1, 2018.