A PhysicalQuantity is a measure of some quantifiable aspect of the modeled world, such as 'the earth's diameter' (a constant length) and 'the stress in a loaded deformable solid' (a measure of stress, which is a function of three spatial coordinates). Every PhysicalQuantity is either a ConstantQuantity or FunctionQuantity. Instances of ConstantQuantity are dependent on a UnitOfMeasure, while instances of FunctionQuantity are Functions that map instances of ConstantQuantity to other instances of ConstantQuantity (e.g., a TimeDependentQuantity is a FunctionQuantity). Although the name and definition of PhysicalQuantity is borrowed from physics, a PhysicalQuantity need not be material. Aside from the dimensions of length, time, velocity, etc., nonphysical dimensions such as currency are also possible. Accordingly, amounts of money would be instances of PhysicalQuantity. A PhysicalQuantity is distinguished from a pure Number by the fact that the former is associated with a dimension of measurement.
| ||CompositeUnitOfMeasure||Instances of this Class are UnitsOfMeasure defined by the functional composition of other units, each of which might be a CompositeUnitOfMeasure or a NonCompositeUnitOfMeasure.|
| ||NonCompositeUnitOfMeasure||Instances of this Class are UnitsOfMeasure that are applied to a single dimension, and so are not intrinsically defined by the functional composition of other units.|
| ||SoundPressureLevelUnit||The Class of all UnitOfMeasure that measure a sound pressure level.|
| ||SystemeInternationalUnit||The Class of Systeme International (SI) units.|