Andy's information is also my recollection of the meaning of LUF:
Seem to remember reading that LUF is a calculation based on D layer
absorbtion. Propagation predictions software giving LUF uses sunspot
number / solar flux and time plus lat / long, but unlike MUF, also needs
transmitter and antenna properties in order to calculate signal strength
LUF is the lowest frequency that will propagate over the skywave path(s)
that is subject to the D layer absorbtion, whereas MUF is a geometrical
consideration only, so doesn't need power and antenna details.
For F layer one hop geometry, a ray is subject to two doses of absorption
from the D layer. Absorption increases with decreasing frequency, which can
be considered as a path loss matter, and is not a sharp cut-off mechanism.
Path loss can to some extent be compensated for by increasing transmitter
power and antenna gain. Even for really high radiated power, there is a
point where only a one hop path is useable (there is twice the loss in dB
for two hop geometry), and even a one hop path can have too much loss for a
low powered system.
The practical impact is that D layer absorption greatly attenuates skywaves
on lower frequency bands during hours of daylight (the sun being the
stimulus of D layer ionisation). Best DX is at night-time.