cover
is how "shielded" the signal is from being totally cut off. cover
140 means that the 140 highest (brightest) values (116-255) can result in output > 0.
sharpness
describes how fast the light fades. 0,98
means that the light fades about twice as fast, but the fading effect is not linear, it is reduced for darker areas.
I would expect this filter to darken and sharpen overexposed images.
Exponentfilter is a fitting name, since the sharpness function uses exponents (pow is the exponent function).