for the johnsons and rancos, that statement is geared for for hvac equipment. they are basically saying to use limits and sensors to cut off equipment and don't rely soley on the thermostats. example if one was used for a drink cooler/walkin fridge type you would use a limit set or made to the lowest temp acceptable and that would be a safety or backup circuit to turn off compressors/fans if the thermostat should fail. same could be said for high temp applications. some fan motors and compressors cost on the thousands and you want additional safety/limit devices to protect them.

vaughn