After the discussion about units, it is probably important to remember the following: data models generally do not focus on implementing every possible option. What they do is instead choose one standard unit for internal storage, and use that.
Meaning: we don't need a unit ontology, or to express e.g. MASS as an entity that has both a value and a unit. We just need to say that MASS is a value of mass in kilograms. If we want a different unit, it is generally because it is relevant to a specific field (e.g. mass in atomic units is called atomic mass unit and values are expressed as such, e.g. 13 amu)
After the discussion about units, it is probably important to remember the following: data models generally do not focus on implementing every possible option. What they do is instead choose one standard unit for internal storage, and use that.
Meaning: we don't need a unit ontology, or to express e.g. MASS as an entity that has both a value and a unit. We just need to say that MASS is a value of mass in kilograms. If we want a different unit, it is generally because it is relevant to a specific field (e.g. mass in atomic units is called atomic mass unit and values are expressed as such, e.g. 13 amu)