In plain language: The lowest usable frequency is the lowest frequency expected to provide the required signal-to-noise ratio on a specified path at that time and power.
Why the station behaves this way
Absorption and atmospheric or human-made noise often set the lower limit. Transmitter power, antennas, mode, bandwidth, and required reliability change it.
A common wrong turn
LUF is not simply the lowest amateur band and does not mean that signals below it are physically impossible; it is a service-quality planning threshold.
What to do at the radio
Model the real path and service requirement, inspect D-region absorption and noise, compare adjacent bands, and use narrower modes or better timing when the lower band is marginal.
Operating boundary
Use current equipment instructions, legal operating limits, the exact signal path, and observed conditions. A connector, software label, transformer ratio, or forecast value does not prove complete-system performance.
