NOx compliance depends upon the design requirements of engines onboard. Compliance is insured by EIAPP ( Engine international air pollution prevention certificate) issued by the manufacturer of the engine. IAPP( international air pollution prevention certificate) issued by class.
EIAPP is issued by the manufacturer of the Engines stating that the engine comply with the NOx requirements. IAPP is issued by the Class after physical verification of the compliance post fitting of engines onboard. The procedure to verify compliance throughout the ships life is stated in NOx technical file maintained onboard which tells you what all parameters you are suppose to maintain on engines in order to keep Nox in control.