Plain-language answer: Optimum working frequency, often called FOT, is a planning frequency below the predicted MUF chosen to provide better reliability as conditions vary.
Why operators care
Operating exactly at predicted MUF is fragile because the ionosphere and model are uncertain; operating too far below can increase absorption and noise.
What to compare on air
Use a current path prediction, start near the recommended FOT, then compare adjacent authorized bands.
The common mistake
Treating FOT as the one frequency guaranteed to work all day.
Forecast boundary
Space-weather indices and propagation models describe probabilities and regional conditions. A usable contact still depends on the exact path, time, frequency, antenna pattern, noise, power, and current observations.
