a. kV is for penetration & contrast
b. mA is the amount of current thus the blackening of the film
c. time is the how long the exposure is
kV is for penetration: Thus, when doing chest x-rays at 100cm instead of 180cm (such as AP view), the kV has to be reduced by about 20%.
kV determines contrast: The higher the kV, the lower the contrast. Thus, penetrated film would have a much lower contrast relative to less penetrated film.
mA is for blackening of film: The thicker the tissue, the higher should be the mA. Thus, if you normally use 6mAs for normal size individual, you will need to reduce this to 4 for skinny individual or increase to 8 for obese individual.
Similarly, if for chest radiograph you use 6mAs, but for finger, it should be reduced to just 1 because of thinner tissue.
Time is how long the exposure is: Use shorter time for chest x-rays but longer time for extremity x-rays.