Oh sorry back to the question. In the absence of anything better, i think its an accurate guideline for the greater majority of people.
BMI is calculated on height and weight and takes into account how much muscle mass and fat a healthy person would be at their age.
There is also the healthy weight range which is given in pounds and kgs not in bmi. It is probably easier to understand than BMI.
Personally i think its good to aim for the middle of the range because if you take your foot of the break too soon, you might go back up too soon. If you aim for the bottom of the range, you might push the limits of what your body will allow before it tries to rebound. That happened to me last time. I am trying to avoid that this time. I know it can be done. I know i can reach the bottom end of the range and stay there but it should be done slowly and carefully and i don't think i need to be that skinny really. Its too much effort.