40 CFR § 86.1802-01 - Section numbering; construction.
(a) Section numbering. The model year of initial applicability is indicated by the section number. The two digits following the hyphen designate the first model year for which a section is applicable. The section continues to apply to subsequent model years unless a later model year section is adopted. Example: Section 86.18xx-10 applies to model year 2010 and later vehicles. If a § 86.18xx-17 is promulgated, it would apply beginning with the 2017 model year; § 86.18xx-10 would apply only to model years 2010 through 2016, except as specified in § 86.18xx-17.
(c) If a regulation in this subpart references a section that has been superseded or no longer exists, this should be understood as a reference to the same section for the appropriate model year. For example, if a regulation in this subpart refers to § 86.1845-01, it should be taken as a reference to § 86.1845-04 or any later version of § 86.1845 that applies for the appropriate model year. However, this does not apply if the reference to a superseded section specifically states that the older provision applies instead of any updated provisions from the section in effect for the current model year; this occurs most often as part of the transition to new emission standards.