Yep, this is simply the lowest setting. It also means that on this valve there is NOT a positive OFF setting. If you need to isolate the rad for any reason, you must remove the TRV head, and fit the manual cap, supplied with TRV initially. Otherwise, the valve will open if the room gets very cold.
Not necessarily fireman. I have both a frost setting and an off setting on mine. I have an off setting, then a frost setting, then I, II and III and finally a full setting. Not that anything other than the full has been used over the last month :o)