I think this kind of thermometer are used in North America and Europe. People used them in driving and at home. So, "in" means in the car or the house. The probe is put outside the window of the car or the house to let people know how cold the weather is outside ("out"). HK people introduced it into FS around three to four years ago. Should be Benliu series of shop sold it pioneer. ($85 to $120, I remember
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) Later people modified them to suit for aquarium tanks. For these modified version/generation : "in" means the reading from the probe you put inside the tank and 'out' means the room temp. Then further improved products come with the min/max temp alarm, or with the min/max temp record of the day.Now, some PRC products are very affordable.
So, how to interpret the meter reading depends on which generation/version you are using.