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A QuantityKind is a kind of quantity that may be stated by means of defined units. For example, the quantity kind of length may be measured by units of meters, kilometers, or feet.

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REFERENCE CARD: SysMLv1 QUDV - basic metamodel
QuantityKind and ValueType example - length vs radius vs radius of tire
QuantityKind and ValueType example - frequency vs clock frequency vs sound frequency
Figure 34: Value types and units for physical quantities
Source
OMG Systems Modeling Language (SysML) 1.6
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About Object Management Group copyright in text extracts quoted from OMG specifications for educational purposes
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SysML keywords
QuantityKind
Unit
Unit::quantityKind
ValueType::quantityKind
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A QuantityKind is a kind of quantity that may be stated by means of defined units. For example, the quantity kind of length may be measured by units of meters, kilometers, or feet.
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QuantityKind is defined as a non-abstract SysML Block defined in the SysML UnitAndQuantityKind model library.
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SysML ValueType adds an ability to carry a unit of measure and quantity kind associated with the value.
A quantity kind is a kind of quantity that may be stated in terms of defined units, but does not restrict the selection of a unit to state the value. A unit is a particular value in terms of which a quantity of the same quantity kind may be expressed.
quantityKind : InstanceSpecification [0..1] A kind of quantity, represented by an InstanceSpecification classified by a kind of SysML QuantityKind, that may be stated by means of units. A value type may optionally specify a quantity kind without any unit.
unit : InstanceSpecification [0..1] A unit, represented by an InstanceSpecification classified by a kind of SysML Unit, in terms of which the magnitudes of other quantities that have the same quantity kind can be stated.
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QuantityKind is defined as a non-abstract SysML Block defined in the SysML UnitAndQuantityKind model library.
QuantityKind, or a specialization of it, classifies an InstanceSpecification to define a particular "kind-of-quantity" in the sense of an "aspect common to mutually comparable quantities" [VIM3-1.2], ...
... a SysML value property is understood to correspond to the VIM concept of "quantity" defined as a "property of a phenomenon, body or substance, where the property has a magnitude that can be expressed as a number and a reference" [VIM3-1.1].
Modelers specialize QuantityKind as done in SysMLs QUDV model library or in a similar manner in other model libraries.
The definitionURI of an InstanceSpecification classified by a kind of QuantityKind identifies the particular "kind-of-quantity" [VIM3-1.2] that the InstanceSpecification represents.
Two such InstanceSpecifications represent the same "kind-of-quantity" if and only if their definitionURIs have values and their values are equal.
The only valid use of a QuantityKind instance is to be referenced by the quantityKind property of a ValueType or Unit.
A Unit is a quantity in terms of which the magnitudes of other quantities that have the same quantity kind can be stated.
A unit often relies on precise and reproducible ways to measure the unit. For example, a unit of length such as meter may be specified as a multiple of a particular wavelength of light.
A unit may also specify less stable or precise ways to express some value, such as a cost expressed in some currency, or a severity rating measured by a numerical scale.
Unit is defined as a non-abstract SysML Block defined in the SysML UnitAndQuantityKind model library.
Unit, or a specialization of it, classifies an InstanceSpecification to define a particular "measurement unit" in the sense of a "real scalar quantity, defined and adopted by convention, with which any other quantity of the same kind can be compared ...
Modelers specialize Unit as done in SysMLs QUDV model library or in a similar manner in other model libraries.
The definitionURI of an InstanceSpecification classified by a kind of Unit identifies the particular "measurement unit" [VIM3-1.9] that the InstanceSpecification represents.
Two such InstanceSpecifications represent the same "measurement unit" if and only if their definitionURIs have values and their values are equal.
The only valid use of a Unit instance is to be referenced by the unit property of a ValueType stereotype.
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