Why LUF is the Lowest Usable Frequency, and like MUF it is defined for a specific path between two specific points at a given time. Below the LUF, absorption in the D layer (which increases as frequency decreases) attenuates the signal so much that it arrives too weak to be usable, even though the ionosphere would still refract it back. Usable HF operating frequencies for a path therefore lie in the window between the LUF and the MUF.
Watch out The choices tied to a 100-mile radius, a 24-hour period, or the past 60 minutes add conditions that are not part of the definition; LUF is path-specific and changes continuously with ionospheric conditions.
MUF and LUF are both point-to-point: work between them, too low gets absorbed, too high skips over.