If you look at the typical product description of a resistance band (Amazon is a good place to see a lot of these), you'll see that nearly all of them give a range of resistance. For example, one band might say 25-80 lbs of resistance while another says 50-125 lbs.
What do these ranges mean and how do I achieve the resistance levels in between? Does the amount of resistance depend on where you're stabilizing the band? If it's 50-125 lbs of resistance, how do I know when it's 50, 75, or 125? What do I have to do to get 50 lbs if I want to do curls? How would I use the same band differently if I wanted 125 lbs for leg exercises?