IfcSIUnit — the unit an IfcMeasureWithUnit or an IfcPhysicalQuantity names.
IfcSIUnitName, e.g. 'SQUARE_METRE'.
Optional
IfcSIPrefix (e.g. 'MILLI', 'KILO'). Absent means the unprefixed SI unit.
IfcSIUnit — the unit an IfcMeasureWithUnit or an IfcPhysicalQuantity names.