An emission factor converts an activity, a litre of diesel, a kilowatt-hour, a tonne of steel, into a mass of CO2 equivalent. Pick the wrong factor and the arithmetic downstream is wrong no matter how careful the data collection was.

Source hierarchy

Prefer supplier-specific factors first, national grid or DEFRA-style databases second, and generic global averages last. Record the source and publication year for every factor used, an auditor will ask.