Vehicles are tested as per their Gross Vehicle Weight (GVW) category. Therefore for vehicles under GVW 3.5 tons, the vehicle is tested on a Chassis Dynamometer and the emissions measured in ppm are converted to mg/km or g/km. And for vehicles having GVW above 3.5 tons, the engine is tested on an engine dynamometer and the emissions measured in ppm are converted into mass-specified emissions of unit mg/kWh or g/kWh. So as the engine has its power rating in kW therefore mass specific emissions are calculated.
Vehicles are classified on basis of Gross vehicle weight of the vehicle(L,M,N) . The lighter vehicles (L,M ) have it in g/km and the heavy vehicles (n) have it in g/Kwhr . So i would suggest you find for what vehicle you want the norms