meaning of “full”
You are correct in saying that FULL can be full on or full off. However it is only logical to understand that on the way up full is ON, and on the way down full is OFF. If the existing temperature is lower than the target then the kiln power is on; if the existing temperature is higher than the target temperature, the kiln power is off. There is no vital point to empahsise. It is a contextual/conditional set of statements that reading of the manual elucidates.