Understanding BMI Print Out

There are a few things there. Fat%, impedance, FFM (fat Free mass) and TBW (total body water) are all calculated by the scale's bioelectric impedance measurement mechanism, which you should be skeptical of. Your BMR is what the scale estimates you burn just existing, without factoring in exercise or what you burn just by going about your day.

Okay, so bioelectric impedance measurements aren't exactly... accurate. The way they work is to put a very minor current into one arm (or leg, or finger) and measure the resistance [aka impedance, measured in Ohms] between that and the second input. They use that resistance to estimate what percentage of your body is water and from there, using your weight, to extrapolate lean body mass vs fat (which has a higher resistance). So it's hugely affected by being well hydrated vs dehydrated, or by having recently exercised, or by your body composition, or by how recently you ate, your heart rate, or the price of tea in China.

http://digitalcommons.wku.edu/ijes/vol4/iss3/2/ <-- Accuracy of Consumer Grade Bioelectrical Impedance Analysis Devices Compared to Air Displacement Plethysmography
specifically quotes

Other studies suggest that segmental BIA provides a relatively accurate estimate of percentage body fat in specific populations such as high school aged children (17, 21), male wrestlers (32) and the elderly (26). At least two studies specifically indicated that BIA measures are not valid in female populations (10, 13) as was found in the current study. It is possible that there is greater variation in the location of fat deposition in females which makes it more difficult to use a single generic formula to accurately measure %BF using segmental BIA models. That same study concluded that biolelectric impedance analysis was not really accurate enough for one-off readings and was better for watching trends in, say, a weight-loss program.

like - here is an article where they found that eating immediately before taking a %BF reading will cause it to drop precipitously - up to 10% over the course of the day - that it takes 7 hours for readings to return to baseline, and that the effect is additive. They also note that the electrolyte concentration in those meals seems to have a significant effect, that the time of day where subjects received the lowest bodyfat reading was different in men than in women, and that body positioning (ie, is the subject lying down asleep or have they been wandering around) seems to have an effect as well. (Full text of the article is here )

So the accuracy of these things depends on how active you are right now, how active you have been that day, if it's the beginning of your workout or the end, how recently you ate, what you ate, your salt intake, your gender, what time of day it is, and how hydrated you are. At a very minimum.

Also, I have some experience with medical electrophysiology (think heart monitors, or sleep studies). Before you put electrodes on a patient, you scrub the skin with alcohol pads and/or abrasive skin prep solutions in order remove dead skin cells/oils to improve electrical contact and specifically reduce impedance between the inputs. If this is a scale, your callused feet are going to up the impedance significantly.

Basically: if you have regular access to one, and you can take regular readings at the same time over a long period of time, it can be used to track trends in your bodyfat percentage, though I would take the absolute number with a heaping pile of salt. (Which... would then lower the reading).

edited to add the fulltext link which I forgot the first time