The concept of density as denoted by "?" is specifically the volumetric mass density. In other words, this refers only to the mass per volume of the substance.
Although physics applications typically use the SI units, density can be denoted using any units for mass divided by volume. For example, kilograms per liter and grams per milliliter can be used for chemistry applications. Other measurements for density include pounds per cubic foot or pounds per gallon in U.S. units.Learn more about Physics