What makes a heater mobile-home approved?

Mobile-home approved stoves have gone through specific testing to show that they can source their combustion air entirely from outside the house. This is required because mobile homes (or manufactured homes) are often very airtight. Should there be a lack of combustion air, harmful levels of CO (carbon monoxide) could accumulate in the house. This is why mobile-home approved stoves have an adapter that hooks-up to the unit and connects to a fresh air intake on the outside wall of the house through an insulated pipe.