29 C.F.R. § 4219.33
Plans may adopt rules relating to overdue and defaulted withdrawal liability, provided that those rules are consistent with ERISA. These rules may include, but are not limited to, rules for determining the rate of interest to be charged on overdue, defaulted and overpaid withdrawal liability (provided that the rate reflects prevailing market rates for comparable obligations); rules providing reasonable grace periods during which late payments may be made without interest; additional definitions of default which indicate a substantial likelihood that an employer will be unable to pay its withdrawal liability; and rules pertaining to acceleration of the outstanding balance on default. Plan rules adopted under this section shall be reasonable. Plan rules shall operate and be applied uniformly with respect to each employer, except that the rules may take into account the creditworthiness of an employer. Rules which take into account the creditworthiness of an employer shall state with particularity the categories of creditworthiness the plan will use, the specific differences in treatment accorded employers in different categories, and the standards and procedures for assigning an employer to a category.
29 C.F.R. §4219.33