BMI is not a percentage, it is a number with associated ranges that are healthy, overweight, or obese. Google BMI calculator and you will see what I'm talking about.
On your scale, there is probably a way to enter in your age and height. Combined with your weight, the scale spits out a BMI.
BF% is a the percentage of body fat your scale says you have. Usually it works by currents being passed through one foot, across your abdomen, and down the other foot into the machine. The current passes through fat, bone, and muscles at different rates. The machine takes these rates and figures out (roughly) what your body fat percentage is.
If you weigh 100lbs, and you have 18% bf, you have 18 lbs of bf on your frame. Make sure you look up healthy BF% for your age. Women generally need between 8-10% to be considered healthy (on the extreme low end). Slight more, 12-14%, to be considered healthy enough to bear children (on the low end). If I remember correctly, anything up to 32% is considered healthy for women, but it's always good to stay away from the extreme ends of the healthy range. The body likes to be in the middle! Hope this helps!
