public interface UnitPresentation
The descriptive form is invariant across Unit Systems. That is, calorie is calorie regardless of the Unit System. But, the internal form may be 4.1868E7*<2, 1, 1, 0, 0> in one Unit System, and 1.0*<2, 1, 1, 0, 0> in another. Therefore the descriptive form is used as the external representation. This makes it possible to save a model under one Unit System, and then load it under a different Unit System.