Plain-language answer: The F1 layer is a daytime subdivision of the F region below F2, most evident when solar illumination is strong.
Why operators care
It can contribute to HF refraction and absorption, but often merges with F2 after sunset and is not a fixed independent mirror.
What to compare on air
Use current ionosonde profiles where available and compare daylight path behavior across nearby bands.
The common mistake
Assigning every daytime single-hop path specifically to F1 without evidence.
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.
