A bulk product, e.g. crude oil, can be characterized by different quantities.
Examples for relevant crude oil quantities are: The quantities gross observed volume, net standard volume, gross and net standard mass, gross and net standard weight, density in air, density in vacuum etc.
Measurements in the field or laboratory result in specific quantity values for various quantities for a certain amount of product. For crude oil, you typically determine quantity values for the quantities gross observed volume, density in vacuum via measurements.
Quantity conversion calculations, based on defined measurement standards, then result in quantity values for the quantities net standard volume, gross and net standard mass, gross and net standard weight which are required for business processes and financial valuation.