The amount of muscle mass (in kg) you see as a result on your TANITA body composition scale, is the complete amount of muscles that you have in your body, together with all the water they contain. So this means that it shows not only the weight of the skeletal muscles, but also the smooth muscles and all the digestive organs, lungs, heart, sternum and so on.

The BIA technology (Bioelectrical Impedance Analysis Technology) cannot yet distinguish the different types of muscle mass, so when BIA devices do show skeletal muscle mass, it is always a calculation based on averages. In order to give you the most accurate picture of the changes in muscle mass without calculations based on assumptions and averages that could make it less accurate, we have deliberately chosen to show only the total weight of muscle mass in our body composition scales.

Since muscle mass other than skeletal muscle does not tend to change, you can consider a long term change in muscle mass values as a change in skeletal muscle mass, so this way you can track the increase or decrease of skeletal muscles.

See more information here: What is Muscle Mass?