#### IfcSIUnit

Natural language names

##### Semantic definitions at the entity

Entity definition

Definition from ISO/CD 10303-41:1992: An SI unit is the fixed quantity used as a standard in terms of which items are measured as defined by ISO 1000 (clause 2).

IfcSIUnit covers both standard base SI units, like meter and second, and derived SI units, like Pascal, square meter and cubic meter.

> NOTE Corresponding STEP name: SI-unit, please refer to ISO/IS 10303-41:1994 for the final definition of the formal standard.

> HISTORY New entity in IFC Release 1.5.1.

Attribute definitions

# | Attribute | Type | Cardinality | Description | A
--- | --- | --- | --- | --- | ---
3 | Prefix | IfcSIPrefix | [0:1] | The SI Prefix for defining decimal multiples and submultiples of the unit. | X
4 | Name | IfcSIUnitName | [1:1] | The word, or group of words, by which the SI unit is referred to. | X
| Dimensions :=IfcDimensionsForSiUnit (SELF.Name) | IfcDimensionalExponents | [1:1] | The dimensional exponents of SI units are derived by function IfcDimensionsForSiUnit. | X

##### Inherited definitions from supertypes

Entity inheritance
Attribute inheritance

# | Attribute | Type | Cardinality | Description | A
--- | --- | --- | --- | --- | ---
IfcNamedUnit
2 | UnitType | IfcUnitEnum | [1:1] | The type of the unit. | X
IfcSIUnit
3 | Prefix | IfcSIPrefix | [0:1] | The SI Prefix for defining decimal multiples and submultiples of the unit. | X
4 | Name | IfcSIUnitName | [1:1] | The word, or group of words, by which the SI unit is referred to. | X
| Dimensions :=IfcDimensionsForSiUnit (SELF.Name) | IfcDimensionalExponents | [1:1] | The dimensional exponents of SI units are derived by function IfcDimensionsForSiUnit. | X

##### Formal representations

XSD Specification
`<xs:complexType name="IfcSIUnit-temp" abstract="true">
<xs:complexContent>
<xs:restriction base="ifc:IfcNamedUnit">
<xs:sequence/>
</xs:restriction>
</xs:complexContent>
</xs:complexType>
<xs:element name="IfcSIUnit" type="ifc:IfcSIUnit" substitutionGroup="ifc:IfcNamedUnit" nillable="true"/>
<xs:complexType name="IfcSIUnit">
<xs:complexContent>
<xs:extension base="ifc:IfcSIUnit-temp">
<xs:attribute name="Prefix" type="ifc:IfcSIPrefix" use="optional"/>
<xs:attribute name="Name" type="ifc:IfcSIUnitName" use="optional"/>
</xs:extension>
</xs:complexContent>
</xs:complexType>
`
EXPRESS Specification
`ENTITY IfcSIUnit
SUBTYPE OF (IfcNamedUnit);
Prefix : OPTIONAL IfcSIPrefix;
Name : IfcSIUnitName;
DERIVE
SELF\IfcNamedUnit.Dimensions : IfcDimensionalExponents := IfcDimensionsForSiUnit (SELF.Name);
END_ENTITY;

EXPRESS-G diagram

`

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