E24 ECE R122 type approval certificate for diesel heaters means that a diesel heater has been certified as meeting the safety, emissions, and performance standards set out in Regulation 122 of the UNECE. This certification is important for manufacturers and users alike, as it ensures the diesel heater meets international standards for safety and environmental impact. ECE R122 Type Approval is mandatory for parking heaters; without this certification, the heater is illegal.