A little late on the answers, but as they say, better late than never.
About BumpH files...
They are greyscale textures that overlay the underlying texture for a map or an object. They are designed at be a kind of 'topographic map' of the surface, where darker parts are low and lighter parts are high. The game engine then applies local brightening and dimming based on the direction and angular elevation of the light source, so as to create the illusion of bumps and hollows.
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About Landlight.mat files...
They control the degree to which an illuminated surface responds to a light source (Sun and Moon), and the degree of reflectance. The relevant parameters are Ambient and Diffuse, respectively. The higher is Ambient, the stronger the difference of illumination between lit and unlit. The higher is Diffuse, the more brightly reflective a surface under given illumination. The default values for both are "1.0".
The same treatment applies for object textures, where their .mat files define Ambient and Diffuse values.
When altering values, take into account the maximum elevation of the Sun, which depends on the load.ini parameters DECLIN and MONTH. The former sets the latitude for the map; smaller DECLIN sets a lower latitude, where the Sun rides higher at midday. The latter also controls the Sun's maximum altude, being lowest in December and highest in June (for the northern hemisphere). If, for example, Diffuse is set well above 1.0 and it works well for a certain map, for another map where DECLIN is lower and/or MONTH has the Sun higher, the higher Sun would result in extra brilliance of landscape reflectance, likely necessitating a lowering of the value for Diffuse.
And so each map could have its own custom Landlight.mat file assigned, named or located in a folder as apporopriate.