You burn a combination of fat and carbs, and you only need to replace the carbs on the bike. The ratio between carbs and fat depends on your fitness signature and the power you produce.

I did not understand why you would take 25% in the calculation. If 25 % is the efficiency, you should divide on 25% to calculate how much you need to eat (as 75% will be “wasted” by your body).

Ideally, the data field does both - it shows how many carbs we estimate you’ve burned through, which you can use as a refuelling guide. If you’re riding an endurance event, you should use the data field to limit your carb usage to no more than ~100g/hr, since that’s about the upper limit of carb intake for most athletes.

As a side note, assuming Gross Efficiency of 25% is probably a bit of an overestimation - a more realistic value would be close to ~19-20%