40 CFR §1039.740

1. (a)Averaging sets. Emission credits may be exchanged only within an averaging set. For emission credits generated by Tier 4 engines, there are two averaging sets—one for engines at or below 560 kW and another for engines above 560 kW.
2. (b)Emission credits from earlier tiers of standards.
   1. (1)For purposes of ABT under this subpart, you may not use emission credits generated from engines subject to emission standards identified in appendix I of this part, except as specified in § 1039.102(d)(1) or as follows:
   2. (2)Emission credits generated from marine engines certified to the standards identified in appendix I of this part for land-based engines may not be used under this part.
3. (c)NOX and NOX + NMHC emission credits. You may use NOX emission credits without adjustment to show compliance with NOX + NMHC standards. You may use NOX + NMHC emission credits to show compliance with NOX standards, but you must adjust the NOX + NMHC emission credits downward by twenty percent when you use them, as shown in the following equation:
4. (d)Other restrictions. Other sections of this part specify additional restrictions for using emission credits under certain special provisions.