20 C.F.R. § 225.15
The Overall Minimum PIA is considered when the employee would be eligible for an old age insurance benefit or a disability insurance benefit under section 202 or 223 of the Social Security Act based on combined railroad and social security earnings. The Overall Minimum PIA is used in computing the social security overall minimum guaranty amount. The overall minimum guaranty rate annuity formula is discussed in part 229 of this chapter. The Overall Minimum PIA is determined under the rules in sections 215 and 223 of the Social Security Act. Railroad and social security earnings are included in the calculation of the Overall Minimum PIA. The Overall Minimum PIA is used to determine the amount which is treated as a social security benefit for the purpose of taxation pursuant to section 86(d) of the Internal Revenue Code of 1986.
20 C.F.R. § 225.15