A milligram is a very small amount (a teaspoon is 5 mg of water). So not a unit to weigh coal in.
mass
It could refer to density, where the mass is measured in milligrams and the volume in decilitres. Or It could refer to concentration in a fluid where the mass of the "active" substance is measured in milligrams and the volume of the solution (possibly solute) is measured in decilitres.
medicine
Milligrams would make more sense.
Mass is measured in kilograms. It can also be measured in grams , milligrams etc.
We often see iron or other minerals in tablets measured in milligrams, or mg for short.
Objects composed of tiny amounts of mass.
The mass of several strands of hair is typically measured in micrograms (μg) or milligrams (mg).
Anything with a combined mass of less than a gram, generally.
Any unit of measure for mass such as 'ounces', 'milligrams', 'kilos' etc.
Mass is measured in grams with various prefixes, including milligrams. So 458mg is 458mg of mass.
Mass is measured in grams, milligrams, kilograms etc. Mass measurements follow the same laws as the standard (Non-US) measurements for distance.