The XML Schema primitive data type "double" is used because it is a single representation that offers the greatest flexibility and precision. However, there is no requirement to store the data using any particular machine representation, and applications receiving tdata in elements of this type may choose to coerce the data to any other type as convenient. The choice below is intended to capture the following: either BaseUnit or the sequence unknown and ConversionToBaseUnit. The sequence is intended to be either unknown or ConversionToBaseUnit, or both. If both are given, the semanitics is that the exact meaning of the unit of measure is not known, but a reasonable conversion formula is given as a guideline. This data type allows the values of the attribute to be either Yes or No. Users may want to extend the list to include other case scenarios, or abbreviations, such as YES, yes, Y, y.