Reference to a unit of measure definition, usually within the same XML document but possibly outside the XML document which contains this reference. For a reference within the same XML document, the "#" symbol should be used, followed by a text abbreviation of the unit name. However, the "#" symbol may be optional, and still may be interpreted as a reference.
Source
<element name="conversionToPreferredUnit" type="gml:ConversionToPreferredUnitType"><annotation><documentation>This element is included when this unit has an accurate conversion to the preferred unit for this quantity type.</documentation></annotation></element>