Mass
The basic SI unit of mass is the kilogram (kg). In physics, mass is not the same as weight, even though mass is often determined by measuring the object's weight using a spring scale, rather than balance scale comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity, but it would still have the same mass. This is because weight is a force, while mass is the property that (along with gravity) determines the strength of this force.
Kibble balance
In addition to measuring "UI", the laboratory must also measure "v" and "g" using experimental methods that do not depend on the definition of mass. The overall precision of "m" depends on the precision of the measurements of "U", "I", "v" and "g". Since there are already methods of measuring "v" and "g" to very high precision, the uncertainty of the mass measurement is dominated by the measurement of "UI", which is the value measured by the Kibble balance.