Here's the description.
Navman's compact dedicated fuel solution (gasoline or petrol) engines only.the f2100 features an instant fuel flow: 2.5 to 130 liters/hour, 0.5 to 35 us gallons/ hour, or 0.4 to 30 imperial gallons/hour, and is suitable for carbureted 2- stroke engines from 50-300hp carbureted 4-stroke engines from 90- 300hp.the minimum flow rate 5 liters (1.3 us gallons) per hour, and trip and total fuel consumption, user resetable, up to 999 units.the f2100 is designed for single engine applications only, and displays volume remaining in tank (provided that the total tank volume has been entered).the f2100 has a low fuel alarm.
The way I read it is that it will tell you what is remaining as long as you tell it how much was there to start