(a) General rule. For purposes of determining whether there is a partial withdrawal of an eligible employer whose liability is abated under this part upon the employer's reentry into the plan or at any time thereafter, the plan sponsor shall apply the rules in section 4205 of ERISA, as modified by the rules in this section, and section 108 of the Multiemployer Act. A partial withdrawal of an employer whose liability is abated under this part may occur under these rules upon the employer's reentry into the plan. However, a plan sponsor may not demand payment of withdrawal liability for a partial withdrawal occurring upon the employer's reentry before the plan sponsor has determined that the employer's liability for its complete withdrawal is abated under this part and has so notified the employer in accordance with § 4207.3(b).

(b) Partial withdrawal—70-percent contribution decline. The plan sponsor shall determine whether there is a partial withdrawal described in section 4205(a)(1) of ERISA (relating to a 70-percent contribution decline) in accordance with the rules in section 4205 of ERISA and section 108 of the Multiemployer Act, as modified by the rules in this paragraph, and shall determine the amount of an employer's liability for that partial withdrawal in accordance with the rules in § 4207.8(b).

(1) Definition of “3-year testing period.” For purposes of section 4205(b)(1) of ERISA, the term “3-year testing period” means the period consisting of the plan year for which the determination is made and the two immediately preceding plan years, excluding any plan year during the period of withdrawal.

(2) Contribution base units for high base year. For purposes of section 4205(b)(1) of ERISA and except as provided in section 108(d)(3) of the Multiemployer Act, in determining the number of contribution base units for the high base year, if the five plan years immediately preceding the beginning of the 3-year testing period include a plan year during the period of withdrawal, the number of contribution base units for each such year of withdrawal shall be deemed to be the greater of—

(i) The employer's contribution base units for that plan year; or

(ii) The average of the employer's contribution base units for the three plan years preceding the plan year in which the employer completely withdrew from the plan.

(c) Partial withdrawal—partial cessation of contribution obligation. The plan sponsor shall determine whether there is a partial withdrawal described in section 4205(a)(2) of ERISA (relating to a partial cessation of the employer's contribution obligation) in accordance with the rules in section 4205 of ERISA, as modified by the rules in this paragraph, and section 108 of the Multiemployer Act. In making this determination, the sponsor shall exclude all plan years during the period of withdrawal. A partial withdrawal under this paragraph can occur no earlier than the plan year of reentry. If the sponsor determines that there was a partial withdrawal, it shall determine the amount of an employer's liability for that partial withdrawal in accordance with the rules in § 4207.8(c).