How does mass differ from weight?
Do you mean muscle mass vs body weight?
Mass if your refering to muscle mass the exercising terminology means the weight of health or strength trained muscles and is based on height ect.
Where weight is just that what you see on the scale. When some one is calculating muscle mass they do so with a gauge that pinches the muscle to determine how much fat is on your body. Most recent example of a group who swears by this is the US military. So you don't punish those who are in really great shape or strong who might have little body fat but have high weights do to working out. Remember toned muscle weighs more.
in wiki words In informal everyday usage, mass is more commonly referred to as weight, but in physics and engineering weight strictly means the size of the gravitational pull on the object; that is, how heavy it is, measured in units of force.