In kg, your mass is "calculated" for you by using your weight
in lbs, it shows the correct unit, and would always represent your weight on any planet
some people like to use pound-mass to avoid slugs because they are somewhat less common
if you weighed yourself on the moon you would see a different weight, if you divided it by the gravitational constant of the moon you would get the same mass you did on earth
for mass using the kg scale on the moon you would have to multiply it by earth G / moon G
