29 CFR §4203.3

1. (a)General rule. A plan may, subject to the approval of the PBGC, establish by plan amendment special complete or partial withdrawal liability rules. A complete withdrawal liability rule adopted pursuant to this part shall be similar to the rules for the construction and entertainment industries described in section 4203 (b) and (c) of ERISA. A partial withdrawal liability rule adopted pursuant to this part shall be consistent with the complete withdrawal rule adopted by the plan. A plan amendment adopted under this part may not be put into effect until it is approved by the PBGC.
2. (b)Discretionary provisions of the plan amendment. A plan amendment adopted pursuant to this part may—
   1. (1)Cover an entire industry or industries, or be limited to a segment of an industry; and
   2. (2)Apply to cessations of the obligation to contribute that occurred prior to the adoption of the amendment.