Any of the systems devised to measure some physical quantity such distance or area or a system devised to measure such things as the passage of time.<br/><br/>The classes of UnitOfMeasure are determined by the member "measureType." Subclasses are not needed for implementation, but their use makes type constraints on measure valued attributes easier to specify. <br/><br/>-- conversionToISOstandardUnit is not null only if the conversion is a simple scale<br/>