Unless specifically defined in the Texas Clean Air Act (TCAA) or in the rules of the commission, the terms used in this subchapter have the meanings commonly ascribed to them in the field of air pollution control. In addition to the terms that are defined by the TCAA and §§3.2, 101.1, and 114.1 of this title (relating to Definitions), the following words and terms, when used in this division shall have the following meanings, unless the context clearly indicates otherwise.(1) Diesel exhaust--One or more of the air pollutants emitted from an engine by the combustion of diesel fuel, including particulate matter, nitrogen oxides, volatile organic compounds, air toxics, and carbon monoxide.(2) Incremental cost--The cost of an applicant's project less a baseline cost that would otherwise be incurred by an applicant in the normal course of business. Incremental costs may include added lease or fuel costs as well as additional capital costs.(3) Qualifying fuel--Includes any liquid or gaseous fuel or additive registered or verified by the United States Environmental Protection Agency, other than standard gasoline or diesel, that is ultimately dispensed into a school bus that provides reductions of emissions of particulate matter.(4) Repower--To replace an old engine powering an on-road or non-road diesel with a new engine; a used engine; a remanufactured engine; or electric motors, drives, or fuel cells.(5) Retrofit--To equip an engine and fuel system with new emissions-reducing parts or technology verified by the United States Environmental Protection Agency after manufacture of the original engine and fuel system.